PM-3209 added a cluster parameter called pauseMigrationsDuringMultiUpdates. When enabled, it will take a separate write path where migrations are blocked for the duration on the write. There is concurrency test to ensure that updateMany/deleteMany work properly with concurrent moveChunk when the cluster parameter is enabled. However, there is no testing around toggling the cluster parameter and how that will behavior with concurrent writes. 

See BF-33262 for more detail. 

Possible implementations:

• background hook that turns the parameter on and off randomly
• concurrency test that toggles the parameter while write are ongoing