Oh yeah I had the same problem. It is a bug in grbl.
If you upgrade grbl to the latest version and run the “Nightly build” version og UGS it fixes the problem.
However upgrading grbl is a huge pain as the online directions leave out a lot of stuff or are just plain wrong.
It took me 3 days to get it working. (more here)
The work around is after running the file hit “pause”, then “resume”. It will “complete” the file and return full control.
If you “cancel” it errors and you have to do a soft rest which works, but cycles power to the steppers so they can shift a bit, causing problems with multi bit carves.