Currently ShardingTaskExecutor::scheduleRemoteCommand adds sessionId to the command before its passed to shards. https://github.com/mongodb/mongo/blob/r3.7.2/src/mongo/s/sharding_task_executor.cpp#L122-L131

This is wrong place for command mutating operations. There are other better places such as
build(Un)VersionedRequestsForTargetedShards

another related issue with this code is that it does not pass the txnNumber: adding this code to sharding_task_executor

            if (request.opCtx->getTxnNumber() && !request.cmdObj.hasField("txnNumber")) {
                bob.append("txnNumber", *(request.opCtx->getTxnNumber()));
            }

breaks retryable_writes.js test https://logkeeper.mongodb.org/lobster/build/3fd956889add4ebb942a9029d1966e5c/test/5aa07efebe07c43cdf26e014?bookmarks=0%2C2203