In v4.4, we started attaching the database version to the write path. A v4.4 mongod can route a write to a v4.2 mongod as a part of a $merge and if this happens the v4.4 mongod will attach a database version. If the v4.2 mongod is stale, it will throw StaleDbVersion and should refresh.

Note that in most cases, mongos is the one to route to a shard and it should always be at the same or a lower version than any mongod(s), so this is only a problem with a v4.4 mongod routes to a v4.2 mongod.