[Discuss-gnuradio] WBFM modulation/demodulation high frequencies highly attenuated

I'm modulating/demodulating a wide band FM signal and I observe that high frequencies are highly  attenuated.

This is the diagram

And this is the spectrum

High frequencies are attenuated more than 40dB over low frequencies.

It seems to happen for frequencies above 20Khz.

Changing max deviation in WBFM Transmit has no effect.

What am I doing wrong?

regards