This only affected a few pages so far (knock on wood) and I've only encountered it with pdf files (so far). Just reporting, in case anyone else has noted something similar, but I have no hopes it will be fixed. But then again, who knows?
The problem was encountered by a user of my website, who alerted me a couple of days ago. The page had been created back in 2016. But I've found the same problem with a page created yesterday. The issue has been tested, and it occurs independent of browser, computer, country, whatever.
I have no clue as to what is causing this, as I'm no expert. Would it have something to do with the fact that files are stored in a different domain? It seems the file name is "blacklisted" somehow when associated with the original page. So, in order to download it, you gotta (1) change its name or (2) move it elsewhere.
It's a freaking mess.