I believe the same issue is causing problems when copying/cutting and pasting in the channel rack as well and it seems that a lot of the time I am unable to undo more than once or twice when FL is rewired and have lost synth lines that I cut instead of first copying and then pasting to another instrument in the in the channel rack.
Apologies if I should have made another thread for the channel rack issue however I suspect it is related to the piano roll issue. Win 10, 64 bit OS & DAWs btw. I am having trouble reproducing this bug as it seems to start happening some time into working on a project however I will screen capture a video of it when I next encounter it
EDIT: it happened again and I believe it only happens when audio is playing although I cannot be sure as this seems to happen almost randomly and when I tried to make a better video of it happening it reverted back to behaving as expected. You can see it near the end of this video. I hit delete and all the notes both selected and unselected dissapeared.