Do you remember how many and what kind of items this smart group contained?
I added my experience and further info in a post in the main thread about this:
Sorry for double posting.
Bottom line: these two Ghost groups were files not groups and had size zero before first being imported to DEVONthink in 2019. So they have been picked up as Ghosts because they were size zero, but this has not happened as a result of the DTTG 3 problem.