This entry https://www.ispotnature.org/communities/uk-and-ireland/view/observation/682002/ was one which had an error on the old site (the location not displaying). Went back to it on the new version of the site whilst working through some old record and the location had reappeared.
However when I try to edit the entry (Edit - Details Tab - [remove the text about the location not being available] - Confirmation tab - Agree and update observation) the system produces an error message "ERROR Failed to update observation: Invalid post data".
So instead of being the No Map No Group Bug, it is now the No Group Bug:
Will try and agree and see if I can make the group reappear
Success: now try editing it ???