Midi dropout/voice stealing issue in Pro Tools 12.5

[benergy]

Recently upgraded from 11.3.1 to 12.5 and most things are rocking right along. However, a maddening thing is happening with regard to Midi Merge recording on Midi and Instrument tracks. Say I'm programming some drums. I play in a kick-snare pattern for a few bars and quantize it as I wish. Now, time to play/merge in cymbals and hi-hats over said kick-snare pattern. Hitting a midi note that coincides with the pre-recorded kick or snare hits causes those hits to not play at all. Makes it very difficult to add midi to existing midi whether it's drums, piano or whatever else. Midi Merge is selected and no midi is disappearing or being overwritten, it's just not playing back when new midi is being triggered over top while recording. Doesn't matter if you have one midi track or 50 midi tracks in the session or if it's Kontakt, Arturia, Slate, whatever other kind of software instrument one might run. Doing the same thing but on an Instrument track fixes the issue in SOME instances, but there are many times where I have large banks of things open in Kontakt and don't care to have 30+ instances of that running. Anyone else having this issue?