Static vs. "dynamic" DHCP Leases and DNS IPv4

i newly installed LEDE 17.01 final, after various experiments with RC and trunk.
i've configured dnsmasq and network as default except IPv4 Subnet range:
And i have two NAS, that i put in the Static Leases part. The NAS are configured to use DHCP.

Problem is: DNS seems to be only working, when the NAS refreshes their DHCP Lease. So the static entries are of no use, after the router is rebooted.

Anybody noticed this and has an idea how to make the static DHCP leases permanently available in DNS?

I have my Synology NAS configured to use a static IP served by DHCP as well. I'm using the NAS as Pi Hole server, so the DHCP server on my router points back to the NAS as a DNS server. The NAS connects fine after rebooting. The only other change I've made is to disable IPv6 on the router and NAS.

Is there a lease time set for the NAS under Static Leases? Mine is blank.

Can you set the NAS network to not use DHCP and manually enter the IP address?

i put "infinite" as a text in it. Will try replacing it with "".
I could set the NAS to static IP, but thought of LEDE as the central point to configure every static address.

leaving it empty instead of infinite leads to the same behaviour.
in the config there's no active dynamic lease.
the static is configured like this:

Hostname  MAC-Address                             IPv4-Address    Lease time    IPv6-Suffix
NAS       00:08:9b:d1:aa:d2(     

C:\Users\benutzer>nslookup nas
Server: LEDE.lan

*** LEDE.lan can't find nas: Non-existent domain

C:\Users\benutzer>nslookup nas.lan
Server: LEDE.lan

*** LEDE.lan can't find nas.lan: Non-existent domain

Server: LEDE.lan

*** LEDE.lan can't find Non-existent domain