Fields crossing octet-boundary not serialized correctly

[senier]

package Test is

   type T1 is mod 2**7;
   type T2 is mod 2**18;

   type M1 is
      message
         F1 : T1;
         F2 : T1;
         F3 : T2;
      end message;
end Test;

Serializing a parsed message should yield the original message:

data = b"abcd"

msg = PyRFLX.from_specs (["test.rflx"], skip_model_verification=True)["Test"]["M1"]
msg.parse (data)

if data != msg.bytestring:
    print(f"Serialization mismatch:\ngot:      {msg.bytestring.hex()}\nexpected: {data.hex()}")

This fails:

Serialization mismatch:
got:      616131b2
expected: 61626364