It seems that VE gives erroneous behaviour with captions beginning with ; at the beginning of a line. It doesn't display ;captions any differently from normal text, and when one hits enter, a new line of caption style is created but there's no telling until I've saved page and bold text appeared.
Topic on VisualEditor/Feedback/Flow
Reported as bugzilla:69689
Support for association lists (aka definition lists, aka what we abuse to get lines of bold-faced text) is bug 37938. Unfortunately, I don't expect to see this fully resolved this year.