This is unfortunately a currently limitation with how libopenmpt formats effects; if the format that the file would get imported as in OpenMPT doesn't support an effect (in this particular case, MTM files get imported as MODs in OpenMPT), then libopenmpt cannot show a proper effect letter. OpenMPT handles the volume column a bit differently compared to libopenmpt so that particular effect isn't a problem there, but other effects in non-native formats will also show up as "?", just like in OpenMPT. In the long run, we really need to find a way to show effect letters for effects that aren't native to OpenMPT's formats in libopenmpt patterns.