I get that using Command-Alt with the Up/down arrows moves selected notes up or down an octave, and that using Alt-Shift moves them up or down a semitone. Both these functions retain any accidentals (flats or sharps) and the correct relationship between adjacent notes when they are transposed up or down.
But when you use the simple Alt + Up or Down arrow both an F# and an F natural become G natural (up) or E natural (down). I’m struggling to think why you would want that. I can see that if you are operating on a single note then that may make some sense in certain circumstances as there is no relationship between adjacent notes - but if you are moving a phrase up or down then that will distort the relationship.
So currently you are restricted to semitone or octave transpositions which is not a big deal - but I have been caught out with the above on a few occasions.