I ran System Restore and restored to a time before some updates were installed yesterday, and all is now well. Things were fine this morning after the updates last night, so by trial and error, I've traced the problem to Bitstream Font Navigator. If I run it and use it to add or remove any fonts from my system (which I did this morning), then the icons go messed up again, and I have to use System Restore again. At least I know what caused it, now!
Surface Pro (2017) vs Surface Pro 4