Right now master-slave uses the fCV to determine if it should assign UUIDs to collections on secondaries since UUIDs are not explicitly replicated in master-slave. Nodes think they're in fCV 3.4 until they clone admin.system.version, so when they create admin.system.version they do not assign it a UUID. Only at that point do they realize they're in fCV 3.6 and start assigning UUIDs.