I think this has been the case for a while but I only noticed due to a new visual look in 3.9 that adds a more prominent border.

badge badge-circle calendar_event_user

Those classes are used to add a little circle beside calendar events on the main calendar.

I'm not sure if these predate the addition of Bootstrap, but bootstrap has it's own .badge thing, and as a result these seem to pick up some styles that they probably shouldn't.

Notably, the padding makes them very slightly not circular.