I did make it work in the end. I don't know why the background is peeking through. Enabling layers for these NSTextViews didn't do anything. If anyone with expertise in Apple UI frameworks knows how to fix it, please tell me. It doesn't do this on Ventura.
Now I can safely forget about this project for 6 more months, until Google breaks their shit on the Android side again.
In other news, Swift is a terrible programming language. Even C++ template errors are more actionable than THIS