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

I'm very new to FCPX so this is basic editing.

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. (PHOTO ATTACHED)

I'm not using any effects or colour grading.

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

Try deleting all your render files by selecting "Delete Generated Library Files" in the file menu. Turn off your background render while you're trying the export and hopefully it will help you out. To me, it looks like corrupted render files which happens sometimes, particularly if your source media is file and it plays back properly inside FCPX.