1 - Create Host Group Set, containing, say, 10 hosts.
2 - Grant Acces for this Host Group to some User Group

3 - Create Host Group Subset, containing one or more of the hosts above.
4 - Do nothing regarding the permissions of Host Group Subset

Automatically, maps with hosts of original Host Group Set stop showing up for users.

5 - Grant Users access to the Host Group Subset and the maps appear again.

I think this is a bug, the user already had access to all hosts in the Set group.