Very puzzling and complex. I have been sent some audio stems to which I am adding midi drum tracks. I made a tempo track by extracting tempo from tapping which I have done many times before. This was a syncopated rhythm so some of my tapping was not accurate enough even after tweaking. I thought a simpler way to correct would be to warp the bars to more closely fit the guitar track. This resulted in the midi hits within my midi drum part copies moving either forwards or backwards within the part. I changed all the copies to ‘Real copies’ in case they were being affected by what I was doing to others and squared them all up to their original times. So far so good but not ideal as it means any adjustments to the drum patterns within a part are no longer automatically changed in the copied parts. Anyway, some more warping was needed but now, as I start warping, my drum parts disappear altogether from the track and I have to undo to get them back. Am I missing something here or is this a bug???
Usually, when I have to add drum parts to already recorded material, I just play along and practice until I nail it. To me, this saves a ton of time, by simply not having to doodle around with markers, measures, warping, and whatnot.
I do have both an electronic drum set, and AKAI style drum pads.
Not having a drum kit does preclude your method and also gives little room for changing pattern feels.
I do now have a theory as to why these problems may be occuring - my midi drum parts may be too long [i.e. longer than the time signature changes which are every bar] - I hope to do some experimentation today but this is limited by the number of songs I still have to mix.
Thanks for your interest though.