If some API is not implemented, I would expect that an app which depends on it would not run at all instead of misbehaving like this. 
I’ve just disabled video acceleration (device=dri) for Audiotube with FlatSeal. As expected, no glitches, but now it hits CPU bottleneck.
Yes, I know. Several FOSS apps I need either only have aarch64 builds in Flatpack, or are developed exclusively for Android.