If they did they could get prepare conflicts which would cause deadlocks.
Test that mapReduce fails in a causally consistent session in 4.2
Docs for SERVER-40938: Add tests that dbhash and map-reduce do not accept a read concern
Split jscore tests that run mapReduce but do not specifically test mapReduce into two
Shutting down with prepared transaction can invariant during collection validation
Deadlock between S lock acquisition on secondary and prepare conflict