Bug: Imported XML transitions aren't attached to neighboring clips
I was doing a deeper dive to troubleshoot the XML export issues outlined in a previous bug report (https://adobe-video.uservoice.com/forums/911233-premiere-pro/suggestions/43998075-bug-audio-exported-from-premiere-pro-2021-via-fin) and discovered another bug.
Theoretically you can use a workaround to fix the export bug, if you have a 23.976033fps file you can replace footage with a 24fps file of the same content, make a new sequence off the 24fps file, and then copy your edits over to it. This allows you to export an xml that works when you import it back into adobe... mostly.
I say mostly because there's another bug that pops up, I believe due to the way different versions of Premiere Pro round the framerates. Based on the math of the ppro ticks it looks like 2019 is rounding framerates to 3 digits after the decimal, and 2021 uses 6 digits. This results in different ppro tick values for the exact same cut in different versions of Adobe.
This in itself isn't an issue, but the way it's interpreted causes a bug in xml imports. Premiere doesn't really respect the -1 in start/end xml tags which indicate a transition is attached to one end of a clip.
Apply a 1 second Constant Power transition (24fps) and these values become
When a transition is applied to a clip, the in/out values change by (for example) half the length of the Constant Power transition.
So if you have an audio transition that's fully shifted to the left, the pproticks being just slightly off between versions can cause it to ONLY apply the transition to the left of the two clips. Because the clip lengths have been changed in expectation of a transition being applied to them, you end up with audio that's desynced from the video. For example if the transition was only applied to the left clip, then the audio in the right clip would start roughly 12 frames too early, because it was expecting the additional 12 frames of audio to be used/encapsulated by the transition.
There should be more protection/bug checking around the -1 start/end values so that transitions that are a few pproticks off from where they should be still generate their transition correctly rather than being detached from one of the two clips. In the previously outlined export bug for instance, you could have erroneous transition values that put transitions in the middle of clips, 30 seconds away from any cuts that might have needed a transition. Premiere pro should detect and warn the user of transitions it can't fix, rather than happily placing broken transitions on the timeline.
Anthony Stahler commented
After some further testing I don't even think this is a framerate rounding issue - if you take a normal 24fps video and make audio cuts and then use a Constant Power END aligned transition it will be broken - I tested in both Premiere Pro 2019 and 2021.
Not really sure how such a major bug has survived multiple years without no one noticing and fixing it.
Anthony Stahler commented
It looks like this bug is primarily, or possibly only, triggered with the <alignment>end</alignment> transition type.
I'll investigate further and report back if I find any way to patch the error in the xml, I've messed with it a little bit and haven't yet been able to force it to attach to both the left and the right clip without turning it into a center aligned transition shifted one frame over. Obviously that's not an acceptable result in many tightly edited situations.