Currently when you play certain non-diatonic notes in the ‘diatonic’ algorithm it will incorrectly harmonize them.
For example, if you set it to the key of C and you play a Bb (the bVII), it will harmonize it a minor 3rd above with a Db (instead of a D natural, which would make more sense as it would keep the harmized note in the key of C).
If it were a D natural it would imply a bVII major chord which would be much more useful.
Same goes for Eb (bIII), it harmonizes with a minor third (Gb) instead of a major (G) which would be in key.
So the bVII and the bIII of the key both need fixing, it would make it much more useful as then you could play any of the 12 tones in an octave and they will all work musically in a given key.