to whether include testing with "stale mongos". movePrimary can currently cause multiple mongos to be out of sync until they refresh their ShardRegistry. So the test could be trying to test a behavior that is known to be undefined.
Our docs already has the proper warning:
If you use the movePrimary command to move un-sharded collections, you must either restart all mongos instances, or use the flushRouterConfig command on all mongos instances before reading or writing any data to any unsharded collections that were moved. This action ensures that the mongos is aware of the new shard for these collections.