Reported here: https://developer.mongodb.com/community/forums/t/writeconcern-is-not-allowed-within-a-multi-statement-transaction/16838/2

Setting getLastErrorDefaults in the replica set config will break write operations in multi-document transactions. Each transactional write will fail with:

{'ok': 0.0, 'errmsg': 'writeConcern is not allowed within a multi-statement transaction', 'code': 72, 'codeName': 'InvalidOptions'}

The setDefaultRWConcern command does not have this problem.