Thanks.
Just for clarity, my main router at home is EdgeRouter X, setup as a DNS Forwader, backing off to CloudFlare. It is also running DHCP with Dnsmaq enabled, so statically assigned IPs are able to be resolved by their host name. System domain is also configured as ‘home.lan’. The Raspberry Pi (PiVPN/WireGuard Server) sits on this LAN.
How is it that server side hostnames resolve if using WireGuard on my iPhone (iOS app) over cellular network, yet on the very same iPhone if I switch off VPN but connect to Opal router running WireGuard tunnel (near identical WireGuard config) it does not resolve hostnames?