Moi.
Had the same problem. Notes played ok until I started to record. After that they occasionally became late into the track and sometimes even paused for a considerablr moment at worst. Tried to play thru USB and MIDI, reset keyboard (M-Audio 49e), install new drivers.... Nothing helped.
Then by accident, I recorded with Reason minimized in taskbar. All ok.

It seems that having the track visible on-screen has an effect on recording.