I have been having the same issue with 2013 on a Windows 7 machine. I have multiple bold attributes in my label legends and when they were imported from 2012, they failed to maintain their bold status.
Some items had their label parts in different locations on the fixture, like feet away from the fixture.
Anyway, I ended up opening each fixture (tedious. very, very tedious.) And turning each item bold again. (which was an *incredible* waste of time thanks to the new vectorworks being even slower than 2012 on certain features, like editing the label legend layout).
Anyway, I would select the items, click bold, click an empty space to de-select everything and then exit the editor. That seemed to work.