SERVER-20057 disabled map reduce testing with more than one mongos because the temporary namespace chosen for concurrent sharded cluster mapReduces on different routers can collide. SERVER-43303 enabled this testing because this problem doesn't exist for the new mapReduce implementation. If this testing runs in a 4.4 mixed version cluster though, the old protocol is still used because each mongos uses the 4.2 binary, which can lead to failures.
Either the testing enabled by
SERVER-43303 should be disabled in mixed version clusters or the way legacy sharded mapReduce generates temporary namespaces should be fixed, possibly by using a UUID as the namespace suffix instead of a concatenation of the current time and an atomic counter.