Hyperpixel4 + pi + animations - video 'scanlines'

I have a hyperpixel4 attached to a pi zero (also tried with pi 3b+). Using Pi Video looper (omxplayer), I’ve been testing mp4 videos rendered at exactly 800x480.

With 24fps animations I’m getting a strange, scanline-like effect. I say “like,” because the line flickering is actually diagonal. It’s as if the screen can’t keep up with the movement in the animation. The diagonal cut slowly drifts along from right to left.

I’ve attached a photo of the line effect, this being an animation of a bouncing moon. You can see the diagonal line cutting through the image, that’s what’s going on. (the writing on the screen is just the screen protector)

I’ve tried:
-lower/higher bitrates for the mp4
-using pi 3B+ vs pi zero
-editing Pi Video Looper config.txt to use exact frame buffer of 800x480

Anyone else finding this problem? Any suggestions?