The problem was the following: I started auditing of entities when there already were users and usergroups in the database. Let's say I was modifying some user's groups. This modification caused in addition of corresponding rows in
UserGroup_AUDtable was still empty. Later when I was querying for revisions of
Userentity, it wasn't able to find related
UserGroupentities since there was no record in
UserGroup_AUDtable about those user groups.