If I’m understanding what you are saying, it sounds like reselecting the stored vertex group is doing what Blender’s design re selection flushing implies – as I described in the second post above – right? If so, then a bug report would be rejected, as this is consistent behavior from that “selections always flush” design philosophy. If anyone can think of a feature request that doesn’t feel confusing/hacky to deal with this problem without undoing that fundamental selection design, please use RCS to suggest it.
Sorry, I know there is real pain that this design causes sometimes, but it would take a large consensus among users and developers to change it at this point.