Sharded cluster fails on mapReduce with output collection when namespace does not yet exist

A Java driver test of mapReduce to a non-existent database exposes a difference in behavior between latest builds and the 4.2 release. We're seeing this error:

{

   "ok": 0.0, 

   "errmsg": "database output-2284299633124 not found", 

   "code": 26, 

   "codeName": "NamespaceNotFound"

on latest sharded clusters. The test succeeds on all released versions of MongoDB.