Mongos often fails transactions that use "snapshot" with SnapshotTooOld

$ python PYTHON-1796.py

First transaction command failed: {'ok': 0.0, 'errmsg': 'Transaction 060cae38-7be3-4240-babe-65739200884c:1 was aborted on statement 0 due to: a non-retryable snapshot error :: caused by :: Encountered error from localhost:27019 during a transaction :: caused by :: Read timestamp Timestamp(1555453221, 10) is older than the oldest available timestamp.', 'code': 239, 'codeName': 'SnapshotTooOld', 'operationTime': Timestamp(1555453221, 11), '$clusterTime': {'clusterTime': Timestamp(1555453221, 11), 'signature': {'hash': b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00', 'keyId': 0}}, 'errorLabels': ['TransientTransactionError']}

Traceback (most recent call last):

  File "PYTHON-1796.py", line 13, in <module>

    coll.insert_one({'_id': i}, session=s)

  File "/home/ubuntu/mongo-python-driver/pymongo/collection.py", line 694, in insert_one

    session=session),

  File "/home/ubuntu/mongo-python-driver/pymongo/collection.py", line 608, in _insert

    bypass_doc_val, session)

  File "/home/ubuntu/mongo-python-driver/pymongo/collection.py", line 596, in _insert_one

    acknowledged, _insert_command, session)

  File "/home/ubuntu/mongo-python-driver/pymongo/mongo_client.py", line 1342, in _retryable_write

    return self._retry_with_session(retryable, func, s, None)

  File "/home/ubuntu/mongo-python-driver/pymongo/mongo_client.py", line 1295, in _retry_with_session

    return func(session, sock_info, retryable)

  File "/home/ubuntu/mongo-python-driver/pymongo/collection.py", line 591, in _insert_command

    retryable_write=retryable_write)

  File "/home/ubuntu/mongo-python-driver/pymongo/pool.py", line 579, in command

    unacknowledged=unacknowledged)

  File "/home/ubuntu/mongo-python-driver/pymongo/network.py", line 150, in command

    parse_write_concern_error=parse_write_concern_error)

  File "/home/ubuntu/mongo-python-driver/pymongo/helpers.py", line 155, in _check_command_response

    raise OperationFailure(msg % errmsg, code, response)

pymongo.errors.OperationFailure: Transaction 060cae38-7be3-4240-babe-65739200884c:1 was aborted on statement 0 due to: a non-retryable snapshot error :: caused by :: Encountered error from localhost:27019 during a transaction :: caused by :: Read timestamp Timestamp(1555453221, 10) is older than the oldest available timestamp.