HELP?? Weird glitch/visual artifact happening on my videos on export. Why is this happening?

I don't see any of this in playback on FCP but no matter how I export (Compressor, Export to Youtube) I seem to get these weird artifacts. Its not happening during transitions, its often in the middle of a clip.

I'm not using any effects or colour grading. I'm very new to FCPX so this is basic editing.

All footage is shot on iPhone 11 Pro Max in 4k 24fps, transferred via Image Capture

I'm using this set up:

MacBook Pro (13-inch, 2019, Two Thunderbolt 3 ports)

1.4 GHz Quad-Core Intel Core i5

16 GB 2133 MHz LPDDR3

Intel Iris Plus Graphics 645 1536 MB

macOS Catalina 10.15.2

FCPX 10.4.8

Editing off of a 256GB external SSD