I have the same problem but in the SCL band after L2 processing. After reading the ATBD I don’t see why there would be such a big difference in the SCL layer but maybe I’m misunderstanding something.
are you sure that both L2A data have the same processing baseline? It might be possible that they have different baselines and therefore different versions of sen2cor were used to generate the scene classification. I have downloaded the current L2A of the T29SND tile (S2A_MSIL2A_20170525T112121_N0205_R037_T29SND_20170525T112434) and the scene classification looks exactly like your example of T29TNE.