I have a short MIDI item that holds only two notes. When I split that item between the two notes, the note in the left (after the split) MIDI item is erased. This licecap shows it, I hope:
Surely this is not as it should be...
__________________
// MVHMF
I never always did the right thing, but all I did wasn't wrong...
I think it's the prefs > media > MIDI > allow trim when splitting... option. I think it's there if you work with really huge MIDI files split many times, because each split effectively doubles the MIDI data and at some point Reaper will drown in MIDI.
It looks in your licecap like the first note is before the start off the item too, so that could be a factor.
EDIT: Actually, I can't reproduce that. Not sure what's going on there.