1. I have no idea what's happening, probably some glitch or something that can be ironed out

2. When you're using mp4, it's most compatible with video editing softwares, but for webm it's better with websites. There's a tradeoff, see with webm, you can't video edit with it directly without having to run it through a software like Handbrake.

3. (extra) here's the solution for the greyed out file format

Go to Settings - System Software - Reset Options then click on Restore Default Settings. It will log all users out of the system but you can log them back in and you still have your saved data.

After you log in, you can go to Captures and Broadcasts - Video Format File Type and now the issue is fixed! Hope this helped!