Hi Lumir_Botur - Yes, I agree with Brian here. This seems to be buggy. There was a change as part of 14.1 CP2 where the relationship between assignee and group was enforced, however, being that this assignee belongs to both the old group and the new group, it should not have blanked it out in my opinion.
I would recommend that you open a support case for this one, state the use case and expected behavior vs. actual behavior and an engineer will open a defect for this if it can be reproduced.
If you dont mind, please post the case number here so we can follow it through and post the solution here as well.
Thanks!
Jon I