Breakthough! I had the thought that maybe the selected font didn't have bold in its character set. I went to check, and it was set to "Default dialog font". I changed it to "Bitstream Vera Sans", and now the correct parameter appears in bold.
@Dennis I'm not sure if "Default dialog font" is the default mapping for parameter info, but I don't remember ever changing it. If it is the default, could you make sure that on the Mac it maps to a font that includes a bold character set?