o/
Firstly, I hope y'all are having good holidays.
Fortunately (or unfortunately), this browser test has picked up what looks like a regression.
It's likely caused by the recent Multi-Content Revision work that was merged into core on December, 21st ([0] onwards). The reason I point to this code rather than the MobileFrontend extension or the Minerva skin is that neither project had a change merged on that date (or the 20th) that changed the way they interacted with the core Revision class.
AFAICT it only affects the Special:EditWatchlist page as the Special:Watchlist (and others) are working fine. Just to confirm the obvious, this is only affecting the Beta Cluster right now.
I'll comment on the change that introduced the regression (from our perspective) [0] and submit a change that fixes it, and I'll review the code backing Special:EditWatchlist to understand how a funky value might be getting passed to a method on the Revision class. If there's a task for the next deployment, then I'll comment that this should probably block it.
-Sam