Well, the bigger question might be why do they like NAT?
If it's about having a single /128 address so they can do ACLs then that's easily fixed by just lowering the CIDR number. (unless you have an ancient version of fortigate on prem, which likely doesn't work with ipv6 anyway).
If it's about not having things poking at your servers through the NAT then the "NAT" really isn't helping anything, it's the stateful firewall doing _all_ the work there and those things are entirely independent systems. -- They're just sold to consumers as a single package.