I noticed that rendered videos contain one or more glitches in the footage where picture quality drops considerably for a few seconds, then returns to normal. The problem appears to be a compression issue, it has been present for at least a year, and probably has a complex causal background.
The drops in quality are not random. If the video is re-rendered, they always show up again and at the same time coordinate for the given project. They are visible in both VLC and Media Player.
I noticed it first when exporting Animate CC animations but it also happens when rendering from Premiere Pro.
The problem seems to be, at least partially, connected to software version. With the versions available at the current time for Animate, Premiere Pro and Media Encoder, I cannot get a glitchless render from my files. Previously I was able to downgrade my software which fixed the issue, but those old but functional versions have rotated out and are no longer installable.
But! Hardware also seems to be a factor because in my desperation I attempted to render the files on a different computer and the end result came out flawlessly regardless of software version.
So to sum up: Computer A puts glitches in videos rendered with newer versions of Animate, Premiere Pro and Media Encoder but renders (or I should say rendered) just fine with older versions. On the other hand, Computer B can render perfectly well with the newest versions of Adobe software too.
Specifics: The problem appears when rendering to H.264 format. Exporting to mov files is okay. Toggling between GPU acceleration and Software rendering provides no fix.
The affected system (Computer A): AMD Ryzen 9 3900X CPU, AMD Radeon RX 5700 XT GPU, 32 GB RAM, Windows 10 64-bit (graphics drivers up to date)
The unaffected system (Computer B): Intel Core i5-4690 CPU, nVidia GeForce GTX 970 GPU, 16 GB RAM, Windows 10 64-bit
I attached 2 screenshots taken from videos rendered from the same fla file with the same settings but performed on different systems.
I’m desperate to find a solution because as it stands now, I cannot make any videos with acceptable quality on my own PC. Thank you!

I'm doing lots of source text changes (via essential propeties) in precomposed text layers using external Json files to change the text. In the majority of these cases everything works fine and I've done over 2000 of these renders this way.

But every once in a while the text layer ignores the json file and just used the default text inside the text pre-comp even through it's all connected with expressions correctly.

This is a consistent issue across multiple versions of After Effects, Media encoder and on both Mac and PC. From 2019, 2020, 2021 and the new beta.

As mentioned before, it's inconsistent in it's consistency though as the majority of the clips work fine. It's never an issue when rendering directly from After effects so it's definitely a Media Encoder issue.

It also doesn't matter if I have 1 or 50 text layers, the issue can still occur.

Any help would be appreciated

It's incredible how the evolutions and updates reduce the capacities and performances of this application which is so useful, necessary and important...

Since a few versions, the updates modify the capacities of settings and configurations of the exports in a very restrictive way, not to say aggressive.
Now it's the top: if you want to export H264 in 50i, you can only do it in "CPU only" mode...
It's the last straw when you realize that the CPU is only used at less than 50% of its potential.

Efficiency and performance do not seem to be your leitmotiv anymore... You were already unable to listen, hear and follow the requests of your users and customers (multi-mono audio and timecode for example), it is sad because we liked your package.

General Lag and unresponsiveness is one of MANY performance issues I've had for the last several versions and keep hoping they'll get fixed. I've uninstalled, cleaned, reinstalled multiple times. Sometimes it improves temporarily, sometimes not. Just sending a project from AE to ME, with ME open takes 30-45 seconds. Once there, clicking on encoding options can take another minute for the panel to open. Cancelling a render, when ME gets to a frame that it gets stuck on (and will remain stuck for 15 minutes) - cancelling that job takes longer than force quitting and restarting. Maybe it's plug-ins I have installed? I spent a day uninstalling one at a time and testing once, and not sure I ever found anything conclusive. If it is the problem, there should be a way for ME to detect that and report the problem to the user. I gave up on ME in 2018 and started using BGrenderer MAX, but with the most recent Mac OS update, BGR is now broken too - until Adobe or Apple can a address a code change that broke it. So now I'm back to my struggle to get ME to work. My AE and ME are always up to date. Sometimes new versions work better, but it's usually temporary. I'm on a 2019 Mac Pro. This issue is not specific to certain AE projects, but it does seem worse on the more complex ones.