For the machines that are reverting back to the imported OU, can you check if the HWID (Unique ID) matches/changes between the copy in the custom group, and the active client record one in the imported OU?
If the HWID is different, then that is the reason the client is swapping back, and you should investigate why the HWID is changing.
If the HWID matches, then I'd suggest grabbing the sylink logs of the endpoints in question.