The only sharded collection which has the config server as the primary shard is config.system.sessions. Unfortunately, the code path that the config server calls to create indexes on the sessions collection would currently invariant if one of the targeted shards was the config server itself. To get around this, index commands do not target the config shard, but as a result, listIndexes on config.system.sessions will not return the correct indexes. This bug existed prior to 4.4 as well, since the old code to create indexes on the sessions collection also did not build the index on the config server.