I have a similar setup with the same issue which I tried to solve/troubleshoot for months
without success. I have opened as well few threads on this list and sssd list. The funny
part starts with sssd 2.2 , where even the uid override which used to work on sssd 1.6
does not work anymore .
The first suggestion was to check if a group name exist in AD for the GID you overwrite -
in my case it was - therefore that didn't help.