by Ken Task.
Guess ... in the new server, you might have to re-create the same groups/grouping even if there are no users in there. OR/AND ... wonder if this has to do with cohorts?
Version 2.2.x: https://docs.moodle.org/22/en/Cohorts
Version 3.2.x: https://docs.moodle.org/32/en/Cohorts
'spirit of sharing', Ken