We have managed to solve it! We identified that the IOS versions were different (15.x, 16.x).
We found that version 16 uses the quic protocol to encrypt the traffic, so the MX is not capable of identifying traffic with adult content, for this reason we did not obtain the desired result.
We managed to block the traffic of version 16 by adding 2 layer 3 rules in the MX to block UDP traffic through port 80 and 443 through which quic travels.