sorry for the late reply... Q3 user here too , and not sure it's the same but have noticed of late occasionally black artefacts rapidly appear and dissapear (not sure they were square for me...)
Anyway , I found a workaround by using Pure LCS weather style (instead of Pure Gamma) ...
try and see if that fixes it for you. (also you can try and disable the new KN5 loader option, which is said to bring about a lot of issues).
ps:
Peter Boese announced a new version of Pure LCS soon (next week) , with which also the Showroom would work (now too dark when using LCS), so if that was the reason you were sticking to Pure Gamma, it will soon be fixed!