I have finally located the culprit. According to the 2018 TUC proceedings:
It is noted that, since release v6p2, the use of the LECLIM subroutine has been uniformed across all modules of the TELEMAC system. In LECLIM are performed, in particular, some general consistency checks geared towards the flow modules. The implication is that the TETAP and ALFAP values are reset to 0 when the corresponding codes LIUBOR and LIVBOR are other than KENT (5) or KENTU (6).
Since BlueKenue sets LIUBOR and LIVBOR to 1 by default (and I did not bother to change them), TETAP and ALPHAP were ignored...
Regards,
Costas