Own DNS server doesn't function when second (external) DNS server is configured
I have a Raspberry Pi with Pi-Hole running in my network and successfully configured it as primary DNS server in DHCP Server settings on my X50. Besides add-blocking, I use the Pi-Hole to configure a couple of local DNS records (for example portainer.home, kodi.home, nas.home). Everything works as expected and the local domains can be reached successfully.
Now, I want to configure a secondary DNS server in the X50 in case my Raspberry Pi and/or Pi-Hole for some reason don't function. However, as soon as I configure a secondary DNS in DHCP Server settings (1.1.1.1, 8.8.8.8, or some other external DNS), I cannot reach my local domains anymore (DNS_PROBE_FINISHED_NXDOMAIN). As if the primary DNS server is not accessed anymore and only the secondary one is used.
Anyone has any thoughts on what's going on and how to solve this?