iMovie for iPad - Green Screen Causing Audio Drift?

I'm using iMovie for iPad with video clips I recorded on the iPad. Before going too far, I want to be clear that I recorded the clips with the format set to Most Compatible (JPEG/H.264) as opposed to High Efficiency (HEIF/HEVC). I was told this preserves frame rate to avoid audio drift issues.

I have a series of short clips - about 80 clips of 2-3 seconds each. When I put them side-by-side in the timeline of my video, I experience no issues of audio drift from the start of the video to the end. But I don't want them this way - I recorded this clips in front of a green screen.

When I set them all as green/blue screen clips on top of a background, the audio drifts at a rate of about 1:60 (1 second drift per 60 seconds of video) - with the audio coming in front of the clips. This is an approximate measure. I have turned off all transitions in my background video (it's a 30 second clip that I have duplicated as many times as needed). Everything seems fine, but as soon as I try to play it end-to-end, the drift occurs. Again, only when using the clips with the green/blue screen setting. The audio was not recorded separately; it is part of the video file itself. I have tried exporting the videos to Handbrake and adjusting them to a constant framerate - no change. I have tried detaching the audio from the video files in iMovie - no change. I have tried playing it upside-down under the light of the full moon - no change.

I feel like there's something really simple I'm overlooking, because this just seems so weird and I can't find anything about it in any forums. Please help!