CompareService::buildUserPredicate assumes that any string that is not a valid IP address or range is a username, and adds it into the query.

Since T245499, a per-target limit is applied, so if any of the targets is an invalid or non-existent user name, the limits for the valid targets will decrease.

This should not usually be a problem, because invalid and non-existent user names will have been filtered out by the UsersMultiselectWidget. However, if the widget is ever bypassed, the CompareSerice should filter out invalid and non-existent user names before building the query.