Can this be a problem related to the time representation in Reaper? I don't know if Reaper has such problems, but Ardour does in Version 5. This will be solved in Ardour 6. I observed this in Reaper only once that a midi note is not playing, so likely there a similar issues (??).
What happens when you shift your midi item a little bit (1/128 note or even less)? Do they play then?
https://www.admiralbumblebee.com/mus...ped-midi-notes