Did you create your MIDI Grove Clip yourself? I've never worked with them and I just copy and paste whole bars rather than "rolling" out a MIDI Groove Clip.
I'm not at my X1 workstation at the moment, but I reckon that, before you convert your standard MIDI clip to a Groove clip, you should slip edit the clip so that it extends to the end of the measure. Once you have done that, right click on the clip and choose "Groove Clip Looping" and when you next roll out your clip is should stay in time.
cheers
andy