Why can I no longer drag a contacts into a Group in my Mac's Contacts app?

Moving contacts into groups has always been straightforward in the past. It was simply drag and drop. However, this function stopped working after a somewhat recent update to Big Sur 11.6.x and the lack of performance is still persistent in the most current Big Sur 11.6.8.

It has been suggested in other threads that that the problem is due to either duplicate contact records, contacts already belonging to the Group of interest, or that the Group is a Smart Group with automated membership rules. None of these cases are true for me.

I drag the contact over a group and the Group name doesn't get highlighted as it should. When I release the contact over a group, it simply springs back to its original place in the list of contacts. When I open the group of interest the contact I attempted to drag there is not listed.

This seems to be an obvious bug in the OS or Contacts application, since I have been organizing contacts into groups quite easily and successfully for at least 10 years prior. It is only in the last few months that this has become a problem for me. The quirky thing is that the problem seems to be intermittent and only seems to occur after performing contacts maintenance (e.g. updating or deleting contacts).

A telltale sign that contacts cannot be moved is that the application performs sluggishly. (i.e. takes a few seconds before it responds to clicks, creation of new contacts or saving edited contacts). Another tell is that the Group Location feature doesn't work. (i.e. Holding down the Option key is supposed to highlight the groups that the selected contact is a member of.) When the Group Location feature does not work, it is also impossible to add any contacts into any groups.