For some reason, <hr>, the wikitext four-bar line, seems to being hidden in Javascript and then displayed, at least in Monobook. I am identifying Javascript as the issue since I (unlike the original reporter) get a FOUC in Monobook. (But not in Timeless or Vector.)
Original report: Monobook separator lines have gone invisible.
I assume this is new behavior for this deploy, as the user in question says he has used Monobook for some time.
hr element has height=0 in monobook making it not display
Able to replicate on testwiki, differs if logged in or not logged in:
https://test.wikipedia.org/wiki/Missing_hl?useskin=monobook&safemode=1