I feel your frustration with this because I’ve run into the same problem and it was the main thing that led me to put some sort of “convert video” action into the plugin, even though it uses cloud convert to do it. I initially wanted to do with ffmpeg client side, and for a while this worked until about 6 months ago, all browsers released an update which made it no longer work. But yeh, Apple are damn annoying when it comes to media playback!
So, if you use the recorder to record a video on an APple device, it will play back on an apple device since the codecs used in Safari are all apple and it just works. If you record a video on something like Chrome or Edge, then it might play back but then it may not. Apple iOS15 made some improvements to media playback and I started seeing this working so first make sure you’re using the latest version of the plugin AND you have iOS15 then let me know how that works.
I take it the video convert action is actually working? you can see the new mp4 in your bubble storage area right?
I can run some tests for you to try and help.