The mysterious part here seems to be that both OptSelectCount=0 and UsedCount=0 for index ID 11, yet you are seeing query plans featuring this index which means that at least one of these counters should have a non-zero value. I do not see an immediate explanation for that (I don't think compatibilty mode plays a role here). I recall there have been some bugs around monOpenObjectActivity. Specifically, there was one bug (#545521) which led to incorrect calues for UsedCount for clustered indexes, but that bug was fixed in 15.0.3 ESD#1 (and therefore, it is also fixed in your 15.5 server). I am aware of other bugs that are still open around monOpenObjectActivity, but as far as I can see there are no open bugs related to UsedCount or OptSelectCount incorrectly not being counted.
As for the clustered index, the MDA query excludes only clustered indexes on allpages tables (that's the predicate "IndexID > 1"). For tables with datapages or datarows locking, a clustered index can have any index ID > 2 (index ID 1 is not used for datapages/datarows-locked tables). This is because a clustered index for such tables is very similar to a nonclustered index while for allpages tables a clustered index is a very special case.
So it is perfectly valid to see a clustered index for datarows/datapages tables appear as a result of this MDA query.