Found the issue but cant fix it. The issue is with how Premiere handles Arri Alexa Pro Res files. I dont have the issue with other media or once it has been rendered to ProRes HQ.
I have just confirmed that Adobe Premiere 2020 14.0.1 Does NOT work with Black Magic Designs products when working from Arri Alexa Pro Res Files. I tried other BMD devices and am getting the same results. Transmit to BMD devices creates flashes of green and playback that jumps forward and backwards rapidly. Repeatable.
Premiere 2019 does work without issue. Something was broken in the transition to 2020 in regards to Arri Alexa Pro Res playback via transmit to Black Magic Design Products.

BMD 11.4.1.

Mojave 10.14.6

Sample here on Vimeo.
https://vimeo.com/388263326/d2a38383fc

I just updated all the programs through Creative Cloud into versión 2020. But Premiere Pro won't identify the installed version of After Effects as the 2020. The first 3 attempts I try to replace a clip for a comp in After Effects it did as intended but I couldn't do anything more, it was freezed. For the next attemps I reinstall both programs and now it shows me the message in the attached image "Adobe After Effects 2020 debe estar instalado para importar este archivo". I am curretly going back to the previous versions to try it.
I have all the creative suite programs installed in a diferent hardrive of the booting system. But they are in the same folder, as well as the Caché folder.

This seems like a really arbitrary limitation -- what if I need to see overlays for stuff on V9 or A9 or above?! Currently I'm working on a sequence with 15 video tracks and 20 audio tracks, and that's tame for a workflow that often involves layers for storyboards, footage, graphics, adjustment layers, vfx, color, etc.

1. Should be able to read overlays from any existing track or all of them




2. Add a "Read From the Top Down" overlay option, that could function exactly like the "Selection Follows Playhead" feature when all the video selectors are turned on -- i.e. the overlay displayed is for whichever clip is under the playhead, highest up in the layer order, and has an active selector.




3. Allow better sizing and positioning options. The increments between font size options are waaaay too big, and because the size is also affected by the monitor it's transmitting to there's just not enough control to get it the right size sometimes. Let me just type in a number to choose my font size.

Also the positioning is difficult to adjust -- only having the three options of Top Alignment, Vertical Alignment, and Bottom Alignment is not enough, and often overlays are cut off because of sizing and positioning issues, especially with clips with long file names -- which are very common in post-production. Allow more control over sizing and positioning of overlays please!

Well. Another thing discovered with Mercury transmit that makes it freak out. UHD monitors... At least that's my guess since this one thing doesn't occur unless mu cintiq pro 16 UHD tablet is connected with displayport to my gtx1080 gpu.

Here's the thing. While the Cintiq is wonderful in many ways. It's just not built to be used for color grading of video. Even my low tier 28 inch 1080p benq works better because of its sheer size. I have color caribrated it with a colorimeter so the representation of colors is decent enough. And the size makes things feel more like the final output.

But as with many things Premiere seems to freak out over my preferred choice. Because when I have the cintiq connected, then the Mercury Transmit engine gets totally confused about what a pixel is supposed to be and the "full screen" output is scaled to a quarter of its intended size and is sent to... Uhm... One screen at random. I never can prefict which. It's trial and error each time I open premiere.

So to recap the issues with Mercury Transmit that I have been stuck with for a few years:

1. If a UHD display is connected and it's not set as the target for Mercury Transmit , then the output is quarter sized into the corner of a random screen.




2. The numbering of the screens in Mercury Transmit seem to be randomised. And for some reason isn't the same as the numbering in windows own settings. I have three 1080p screens plus one 2160p screen. Again. Trial and error is the only solution for me here.




3. When choosing a screen for Transmit, the only way to see the result is to move premieres window and flail it around all screens to trigger the actual move of the output.




4. If I update the audio drivers (standard motherboard audio), and have Transmit enabled... Premiere crashes without an error message.

In all, I have now given up on Mercury Transmit as a concept because it just isn't worth the hassle. Most of what I do is formated for Scope video anyways so I have now resorted to disabling Transmit and move the program monitor where I want it and manually resize it to full screen (why can't we have a full screen button for floating windows in premiere and after effects?!) The output is still a bit wonky since 100% size is translated to 25% display scale for the program monitor so I have to use "fit" instead and just hope that it is close enough to get pixel accurate detail.

I really thought that Adobe was trying to get me to not think about the software and just edit my video... Why make it so ******* us?

Ok. So I opened premiere pro today and... lo and behold. Yet again... Mercury Transmit decides to not play along. I don't get an error message or anything. Not even a hint of any other trouble inside the program. It just does not... well... transmit anything to the screen I tell it to.

I have disabled the immersive damned environment (Why is this not disabled by default by the way?!!!), I have set it all up all according to what is supposed to work. What has usually worked. I enable mercury transmit for only one screen, number 1 as that one is the one that is connected to the physical screen in question. But it's like Premiere (and after effects too) just don't care to send anything there.

I have checked for updates both for my graphics card (nvidia gtx1080) and for the adobe programs in question (After Effects and Premiere Pro). I have checked that Sonic Studio is disabled as I know that one has had a history of not cooperating with Mercury Transmit. I have rebooted the system multiple times. I have rebuilt the adobe folder in my documents...

Please... I do not know what to do here. The functionality just sometimes decide to **** out on me and I am left none the wiser as to what it is that Transmit finds so offending. It just up and decides sometimes to not work sometimes.

At least give me an error message or something. Please!

And yes... the screen in question works just fine with windows 10 in general. It's just Transmit that refuses to acknowledging its existence or something...

I cannot select my second monitor as Transmit output, every time I do it un-ticks itself when I leave the preferences making it unusable. From time to time, the transmit output goes fullscreen onto my main screen, covering the interface and rendering Premiere unusable for a few seconds...

Secondly, when I updated to the new version of Premiere, I also updated my AMD drivers which disabled OpenCL. Unfortunately, the Compatibility Report option didn't detect any problem when I thought it was supposed to guide us as to which is the latest compatible driver, leading me to spending hours searching and installing compatible drivers...

I'm currently working with a Blackmagic setup, but it would be great to have Premiere more robust in allowing various scaling options when using Mercury Transmit.

As is more and more the case: people are producing a wide range of aspect ratios (1:1 and 9:16 for social media are most common), but to display those on a standard 16:9 monitor means seeing nothing but black because it's not enabled, or seeing a stretched image because scale-up / scale-down is enabled.

Please allow us to set what the monitor is (perhaps?) and let Premiere add letterboxing (pillar bars or otherwise) to correctly display the aspect ratio of the media on the display that is chosen.