At the end of a $out operation, the temporary collection is renamed to the output namespace. For timeseries collections it is the buckets collection what gets renamed.

It would be wrong that this rename causes a buckets collection test.system.buckets.foo to appear when there's already a test.foo sharded collection. To prevent that, _internalRenameIfOptionsAndIndexesMatch has a check to ensure that the destination collection is not sharded. However, the implementation (here on master, andhere on v7.0) of it is wrong for timeseries: It only checks that the buckets collection is not sharded, rather than the undelying view namespace