pfSense

In /sbin/dhclient-script : add_new_resolv_conf(),
it says:
@ # Add a route to the nameserver out the correct interface # so that mulitple wans work correctly with multiple dns # also backup the nameserver for later route removal
echo $nameserver >>/var/etc/nameserver_$interface
$ROUTE add $nameserver -iface $interface

Unfortunately, adding these routes will cause problems if the nameservers are not in the same network of that interface.
For example,

dhclient: New IP Address (re1): 172.16.26.99
dhclient: New Subnet Mask (re1): 255.255.255.0
dhclient: New Broadcast Address (re1): 172.16.26.255
dhclient: New Routers (re1): 172.16.26.254
dhclient: Adding new routes to interface: re1

will add some direct routes like

172.16.3.95 1a:8b:41:87:38:99 UHS 0 0 1500 re1
172.16.3.96 1a:8b:41:87:38:99 UHS 0 0 1500 re1
@
, where 172.16.3.95 and 172.16.3.96 are the nameservers.
However, these servers are not in the same network and need another router to reach.
So these servers become unreachable.