Existing code for ungrouping did not correctly handle user counters:
- counter for the group was not decremented
- counters for containing nodes were not incremented
The latter resulted in losing some nodes after orphan cleaning or several save/reload cycles.
The bug did not have destructive consequences until recently,
because it was compensated by another bug (fixed in rBe993667a46c2).
Maniphest Tasks: T74665, T74682
Differential Revision: https://developer.blender.org/D7332