@CptViraj noticed today that horizontal rules (<hr>, ----) are not appearing on Commons. The ---- markup is still creating a <hr> tag, but it is invisible.
I also noticed that redlinks were looking a bit off, and suspected CSS trickery. I noticed that the height for hr tags was set to 0. Through process of elimination, I was able to determine that the skins.vector.styles.legacy module was causing the offending CSS:
This occurred today on Commons, but isn't broken yet on enwiki, so it's likely that the patch that caused it is from 1.36.0. I wasn't able to determine exactly what commit or even what file is causing this CSS rule.