Anyone who has worked with SLOs in Service Manager knows that they are scoped by using a Queue that allows you to set the criteria. You can use combination classes as the target class for a queue. Using a combination class or more accurately a type projection allows you to set the criteria of a queue to the property of a related class. For example, you could define a queue based on the Affected Users Job Title.
However, it would appear there is an undocumented (at least I’ve not been able to find anything yet) limitation when using combination classes. If you specify anything deeper than one level, then the queue stops working altogether. For example, if the Work Item is an Incident, then the Affected User would be the next level (level one), and Target Address of Subscriber Address (User email address class) would be level two. Regardless of what you configure the queue doesn’t obtain any members, even though it should.
It appears to be the same behaviour in System Center 2012 R2 and System Center 2016.
Hopefully I’ll have an answer for this so check back soon. In the mean time, make sure you aren’t setting criteria below one level for any type of queue.