We have seen the same issue after upgrading from 2.5.2 to 2.6.0. The first VLAN in the configuration file doesn't have the problem, all the VLAN after have the extra filename options. It is odd that the filename is different. I haven't found where it is getting that option from.

subnet 192.168.240.0 netmask 255.255.252.0 { pool { option domain-name-servers 192.168.243.254; deny dynamic bootp clients; failover peer "dhcp_lan"; filename "legacy.donotuse"; range 192.168.240.80 192.168.243.249; }

So what does the config on pfSense look like vs your external server config? There must be some difference in the formatting or naming of the option to explain what is happening.

Look in /var/dhcpd/etc/dhcpdv6.conf