So sorry for the mouse issue, I saw it in few other videos and sometimes in my own playthroughs and to be honest, I have no idea what causes it either, but will try to locate it, thanks and wish you all the best!
All good! At the top of my head I would say there's another player camera trying to control the mouse position at the same time as your main one, which creates a conflict, both probably being in an Update state.
Try to stripe down anything that might use your camera, maybe even the cutscene animations since they are positioning your camera in a certain way; start from scratch by using a simple mouse camera movement from Unity then try adding back bits and pieces of your current camera scripts and see if there's a line of code that might trigger this issue. That's all I can think of for now; wish I could help more!
Wishing you the best of luck! 🤗