Wallpaper Not Showing Behind Mission Control/App Exposé or resized Window on only one Desktop
Still not sure why it did that, but resetting the NVRAM and PRAM fixed it for me.
Here's how to do it: https://purplecomputing.com/tech-how-to/how-to-reset-nvram-or-pram/
Or to make it simple. Restart your Mac, and hold cmd + option + p + r until it asks you to log in.
EDIT: It has returned. This is now REALLY bothering me