Yeah it appears to just be that for whatever reason, steam overlay recording (or whatever it's called), just never stopped recording despite the SSD being completely full
So Ideally the solution here would be to implement some sort of disk fill limit for everything outside of the OS so that if you fill it, the OS won't stop being functional. It does warn you on desktop mode that the disk is almost full, but if you just fill it faster than you get to desktop, then this happens.