Alright, I think I fixed the problem. What tipped me off was this thread:
I had no idea that Unity was limited to 4gb filesizes, absolutely no idea. Since most of my game exists in only one scene, the RESS file for that scene was 4gb in the final build. I checked back to previous builds I had made, and I noticed that RESS file slowly climbing towards 4gb in each build, and when it hit 4gb, that’s the build that had this issue.
What I did was select a group of ~25 textures that weren’t terribly important, but they totaled around 100mb. I set their quality to low and checked ‘crunch compression’. After building, the RESS file was only 3.93gb, and this graphical problem was gone. Crunching some textures to fix this problem is probably just a bandaid solution, but apparently 2020.1 removes this 4gb limitation so that’s good to know.