They are, in fact, supposed to maybe or maybe not work, or do different and unexpected things on different platforms (On Windows XP they actually work minus the wrong GUI colors), and that's why they are undocumented and no one knows they exist.
I can't remember exactly why; just that it was the Windows 7 era and there were multiple gfx issues no one knew how to fix.
allegro actually converts all the 8-bit to 32-bit internally before displaying it.