https://bugzilla.kernel.org/show_bug.cgi?id=207901
--- Comment #28 from Lyude Paul (lyude@redhat.com) --- (In reply to Ilia Mirkin from comment #27)
DP -> HDMI is, most likely, just passive, so the displays are being driven as HDMI rather than as DP. DP is the problematic protocol...
Hi-so, my first guess then might actually be a bad DP adapter. In my experience DP cables (usually the full-size ones, but I've had it happen with one or two passive adapters as well) have a bad habit of breaking really easily. One time I actually spent a week trying to fix a bug that'd I'd already tried troubleshooting by replacing the DP cable, only to eventually find out the cable I replaced it with also magically went bad.
Mind posting one more log with the same parameters as last time? I probably can't tell for sure but usually you'll see a lot of link training failures or timeouts when the cable has gone bad (but sometimes the symptoms are a lot more sinister...). As well, I'd also try another adapter of course.