I think I have discovered a bug in the MIDI Overdub mode - steps to replicate:
Insert track from template - Playtime
Insert virtual instrument on new track - ReaSynth
Track recording settings (Track 2) - Quantize track MIDI recording (1/16)
Open Playtime (ctrl-shift p)
Right-click Group 1, use selected track as group (Track 2)
Double-click slot 1, group 1, to new create 1 measure clip
Double-click clip 1, insert notes in MIDI editor, close MIDI editor, exit Playtime Edit mode
Open clip 1 properties - Length is 1.0.0
Open MIDI item properties - Length is 4.0 quarter notes
Close all properties windows
Right-click clip 1, select MIDI overdub, let it run for one or two bars, press space bar to stop
Open clip properties - Length is 1.0.0
Open MIDI item properties - Length is 4.1 quarter notes - BUG !
Subsequent MIDI overdubs alter the length of the MIDI item by random amounts. On playback, the clip is no longer sync to tempo due to the incorrectly sized item (looping is offset by .1 or more qn per loop).
Also, I do not quite understand how to stop MIDI overdub and resume playing the clip. I expected to be able to press the clip launch button to stop overdub and return to playback mode (like normal record mode), but this doesn't happen. It appears that the only way to resume normal playback of the clip is to stop REAPER's transport completely (Space), then relaunch clip. Is that the intended behaviour? It would be really cool to stop overdub and restart clip (according to settings) just by pressing the clip launch button again.