When dragging a RE area with trim contents behind over a bunch of grouped items then the new items on the right belong to their new group, which is the expected and correct.
When doing the same thing but instead of copy dragging, one copy pastes, then the new items on the right are grouped with the old items on the left, which is not correct behavior and not expected.
See gif showing first the correct behavior and then the buggy one:
Action used for copy: "Edit: Copy items/tracks/envelope points (depending on focus) within time selection, if any (smart copy)" [41383]
Action used for paste: "Item: Paste items/tracks (old-style handling of hidden tracks)" [40058]
But the same happens with any Paste action.