Contact 'Groups' in Contact app disappeared in Big Sur

When I upgraded to Big Sur, another issue I had was that only some of my contacts transferred over, and although I had them all assigned to different groups, and always kept the 'group' list visible, this is no longer the case. Where has it gone? I seem to have the option to create a new group, but not to see them.

I re-imported my saved contacts from a previous time machine file, and the missing contacts did reappear - but still no groups. Any suggestions - has anyone else had this? I have nearly 2,500 contacts, so viewing them through groups was invaluable.