It also desyncs with the Final Doom exe and Chocolate Doom (at least, the old version that I tested it with). That actually happens on map22, which makes me think that something goes wrong in map21 but the various ports handle it differently - something that can happen with overflows. However, the standard overflows are not detected. Anyway, Crispy seems to be doing something odd here, and it bears investigation.   Edit: here we are: