Entry in hostnames file changes after reboot

I’m running DynDNS to link my public IP to a FQDN.
I’m running services on a NAS with portforwarding configured.
I disabled the “NAT loopback” option in the forwardig rules, since this does not seem to work at all.
Instead to be able to access the services on my FQDN on both in the inside and the outside, I added the FQDN to the hostnames file (via LuCI) pointing to the internal IP address of the NAS.

All works fine, except after a reboot the NAS IP address I assigned to the FQDN in the hostnames file changes to the internal IP address of the router.

I thought entries in the hostnames should were static.
So why does the system change an entry in the hostnames file after a reboot ?