I've already tried the method you mentioned. But i think there is a bug in pfSense.
What i want to set is
Primary dns 192.168.0.1
Secondary dns 8.8.8.8
Note pfsense ip address is 192.168.0.30
When i set
primary dns 8.8.8.8
Secondary dns 192.168.0.1
Dhcp settings are right this way. And dhcp clients get the correct order from dhcp server
But when i set what i require
Primary dns 192.168.0.1
Secondary dns 8.8.8.8
Clients get
Primary dns 192.168.0.1
Secondary dns 192.168.0.30