Strange black dithered shadows when painting masks - caused by latest update

I've been using Photoshop without this problem for over a year, and now the latest update has caused rectangular dithered shadow patterns to appear while I paint in masks on light areas. Could it have something to do with how tiles are being updated? I increased tile size to 1028k from 1024k but they still appear.

Is there a way to revert to the past version of Photoshop because Adobe probably won't fix the problem they caused.

I've used Photoshop since version 4 and I can't recall too many updates that broke things like this. So glad to be paying monthly with these wonderful updates with the subscription model /s