A clarification:
if I use Reaper without modifying anything in my hardware connection when I press the synt key I can see at the same time that the key in the virtual keyboard is lowered. So the problem is definitely in Biab.
What can I check in Biab?

Biab, however, correctly records the midi I'm playing but it does not make me feel as I record the midi of "direct monitoring" what I play and the virtual instrument I'm playing.