Definitely regressions in the last few months since I last tested it, I'm adding this to my short-list for this week.
Its because the media object changes for figure. The element is now inside an unsized span.
Change 966911 had a related patch set uploaded (by TheDJ; author: TheDJ):
[mediawiki/extensions/TimedMediaHandler@master] Repair iframe mode of video, broken due to <figure> changes
Yep, that does it. :D Good catch!
Should be safe to backport the CSS fix to production branches, or we can let it roll out with the coming trains.
Change 966911 merged by jenkins-bot:
[mediawiki/extensions/TimedMediaHandler@master] Repair iframe mode of video, broken due to <figure> changes
Its been broken for weeks, i think we can wait a couple more days for the train to roll ;)