Since I'm a UI slut(lol) I came across this and was in the SlimUI Discord to troubleshoot and after testing it seems to be GSE causing it after loading each Addon separately. Notice the Unit, Pet(square model frame), Target Unit Frames and Aspect of the Cheetah icon before and after. Those are all done with Weak Auras.

As you mentioned its actually STUF that is broken. Once the updated Libraries are installed they expose problems in STUF

Closing this as there is nothing at this end I can do about it.