That there is always some slight loss due to glass elements compared to the theoretical specification is out of question, but there are definitely individual cases in DXO where F-stop and T-stop are identical.
It is possible that the f-stop at Zeiss was specified somewhat more conservatively, so that it is a theoretical F1.9 in real terms.
The Canon RF 135/1.8 was measured with T1.9, I don't think we will see worse T-stop values with the Plena.
