Hi,
I've got the exact same issue, using a pad (BopPad).
Record in Reaper = many notes skipped
Record in DigitalPerformer = no problem
For some years i've made my (pad) recordings in DP and exported the midi tracks in Reaper.
It works but it's not exactly the most efficient way of working!
The obvious issue is that with a pad you can send some Notes On not immediately followed by Notes Off.
It's absolutely not linked to the VI used (it keeps skipping even without VI on the track/session).
It works fine in DP (and others DAW apparently) but it doesn't work in Reaper!
I hope that the problem will be solved eventually (it's an old bug…) BUT today I've found a better workaround!
I use MidiPipe (MacOs only)
http://www.subtlesoft.square7.net/MidiPipe.html
The settings are:
In MidiPipe
Midi In = BopPad
Monophonic
Midi Out = MidiPipe Out1
In Reaper
MidiPipe Out1 as midi input
And now it works as it should be! I can record my pads directly in Reaper!
Hope it helps.
And hope that the devs will fix that.