Hey there! MIDI is not a save format, it’s an export format. While Bosca tries its best to convert to and from MIDI, it’s impossible to do this flawlessly as its capabilities are different from Bosca’s. Generally export to MIDI should result in a valid MIDI file. Import from MIDI is available, but it’s far from perfect and not every MIDI file can be reasonably imported into the app because.
You should use Bosca’s own format for saving your work, and use export formats when you want to take your music out of Bosca somewhere else.