Exporting a seamless looping video nearly impossible?

[TC5]

I have video loops that are perfectly seamless, but on export from Premiere Pro via Adobe Media encoder, they fail to loop seamlessly when played back with Quicktime. The videos are ProRes 30p 4k, exported to H.264 30p 4k .mp4 files.

This question was discussed in this previous thread:

https://community.adobe.com/t5/premiere-pro/video-not-looping-seamlessly-after-export-from-premiere/...

Info from that thread was helpful to zero in on the problem but I find I get inconsistent results.

When the movie is open in Quicktime, using the trim edit I can see that the end of the clip has either one or more black frames at the end, or extra frames (which are identical to frame 1). Once I trim them out I get a better loop. 1 out of 4 tries on different clips gets me a seamless result. On others I still get a slight timing glitch or brief "light flash" at the loop point.

For a start, a 30p movie reads out as 100 frames per second or something in QT (WTF is that about?)

Each AME exported movie seems to result in slightly different end frame irregularities.

I tried Handbrake for export and the results weren't any better.

Anyone have any solution for this?

Many thanks for any input!

[TC5]

Seems the issue is when playing in full screen (on MacBook Pro). After exporting again using Handbrake the loop works as expected on playback in QT but not when playing the video in full screen, where there's a noticeable "white flash" glitch at the loop point.