The issue reproduces for a LDAP group with 1716 members which didn't properly synchronize with the XWiki Group.
As Denis Gervalle investigated:
AD has a limitation on the number of attribute values returned by a query, and require that a range value is used for attribute having more than 1500 values.
XWiki does not seems to support properly group of that size, this is a limitation of the product.
The retrieval of group members is done with a single query, similar to:
and to retrieve properly member of a large group, the following would be needed:
Using ldapsearch, it automatically limit the range to 0-1499, but without any range, apparently no value of attribute member is returned.
As a result, the group is seen empty.