Just a guess: the two apps on my device that use OSM maps (online or offline) don't show junction numbers either.
Maybe MF simply renders OSM maps to fit their app without adding to them - after all, at least using TTS voices, the numbers are announced several times before one reaches an Exit junction.
I just had a at OSM online and it does include junction numbers but it seems to include a local name that we don't generally know or use when travelling on the motorways.