The config server is the DB-primary for the config DB, which may have config.system.sessions as a sharded collection, therefore it should have a chunkless entry on its authoritative shard catalog (config.shard.catalog.collections). However, it's currently missing when a cluster is created directly on FCV 9.0. This seems to happen because the creation is routed to the alphabetical first shard and not the configsvr itself.

On FCV upgrade it's already correctly handled in SERVER-129001.