I'm using a font which is a TTF with an embedded bitmap, and empty outlines for all the glyphs. SFML renders the glyphs correctly (shows the bitmaps, and not the empty outlines), but if I try to use an outline with this font, it appears to outline the empty font outlines rather than the bitmaps, producing squares behind the text instead of an outline.
Here's an example with 2px black outlines:
If I use an all-outline font, the outlines are correct. I have not tested with pure bitmap fonts because I don't have any that are compatible with SFML.