I'm mot sure about, since when the behaviour regarding group and assignee selection in a ticekt was changed,
I thinks it's since 14.1.03, but at least since 17.0.
In former times we had the behavior, that when the group of a ticktet is set and you want to select an assignee,
you have to select a member of the current group, so for example auto_complete looks for members only.
But one was able to "overwrite that rule" by going to the assignee selection list, doing a second search.
Today, we have the same behaviour when one wants to change the group of a ticket when the assignee is already set.
You are only able to select a group, where the current assignee is a member of.
To transfere the ticket to a non related group you first have to "delete" the assignee, to be able to select any group.
I see both behaviours (old and new) valuably, but alreday heard from customers, that they don't like the new behaviour and asking for rebuilding it to the old one.
So, long speech, short idea:
Make the new behaviour optional (by Option Manager for example), so one can decide which transfere logic maps best to his ticket handling processes.