Final Cut Pro X - Export is pixelated for action scenes

Question

Final Cut Pro X - Export is pixelated for action scenes

Weird question here. I'm doing a fun fan-edit of Infinity War and Endgame, to try and fix some story/pacing issues, and to get better at editing. I've used digital copies of the movie that I own, and have the edit how I want it in Final Cut Pro. Admittedly, I'm pretty new to editing, and some of the technical settings are confusing to me.

However, when I export the file, during random moments, the scene will turn really weirdly pixelated/blocky. It seems to happen most when there are large action scenes, and it's only about 5 minutes total of the 5 hour beast that I ended up with. Here are two sceenshots so you can see the difference between the export and the FCPX footage:

Here are the project settings:

Here are the export Settings:

I thought maybe something happened weird during the export, like my RAM of CPU suddenly got filled up, but I repeated the export with nothing else running on the computer, and the same thing happened, during the same scenes. I'm really scratching my head here, can't figure out what might be causing it. Here's what I'm using: