@splatts - for the time being, I recommend you downloading video-compression app. You can use any app. I use this to compress video - https://apps.apple.com/in/app/video-compressor-reduce-size/id1135183644. Once video compression is done, try uploading the compressed video.
Also, use the latest version of the app - https://testflight.apple.com/join/0tipqwsZ.
It's 1.0.1 (24)