Hey Jack8094,
Thanks for the reply and additional info.
Let's have you save the video to the Photos app, then play there to see if the brightness is back to normal. You'll just need to tap on the down arrow to the right of the video. Then, go to the Photos app to test playing the video.
Since the Messages app can have SMS/MMS (green bubble) or iMessages (blue bubble), which type of message are you receiving when this happens?
One other thing to check, at Settings > General > About > Which "Software Version" are you using? The latest is iPadOS 15.3.1.
Thanks so much, This info will help with the next steps.
Cheers!