Also randomly come across this just today
Got to be to do with google as I added my Gmail account to Mail last week and now I’m noticing it
It isn't Google, and I am really thinking it isn't Apple itself though I noticed it does happen ONLY when you do have those two things linked!
I just went into my contacts because I noticed a person today that had all caps still, but it's been a LONG time since I've been seeing any since I fixed many many of them.
After un-syncing Outlook on both my Windows PC and my phone and my MacBook, I have not had any new all capital letters ever appear again.
It has SOMETHING to do with the Google -> Apple -> Outlook sync flow -- somehow something gets messed up and caps the labels on the way.
It could be something else I don't know of but that is working for me so far.