Like it says. I think that's one of the "Ubuntu" fonts but that's just a guess: what I do know is, it's not the font that it's been told to use and that everything else on the system (caja etc) correctly does for all of their context / popup menus.
The bug isn't present in 16.04, so it's probably from 18.04 and the move to GTK3, but I can't bisect it further than that. The reason I'm posting here rather than just going straight to launchpad is in case someone has a workaround for it that can be used in the interim. (Bed now, launchpad tomorrow).