Im trying V3.15 today with an existing show created on V3.13 where I used to have a midi in list of notes. I went to the midi map editor and I seemed to have lost all the previous config...The current map says - Legacy_midi_netnum_1 and it's empty. Is this normal behaviour?
No it is not normal. When an older show file loads on 3.15 it loads the older data into the new midi map format where you should see the notes displayed. Because each midi map needs a name it WILL create a default name for you like the one you described.