Glad you got it working!

My hypothesis is that it was DNS (channeling Jeff Geerling here). Since Pihole is your DNS (makes sense), it may have recognized that address as its own and given you its IP. By resolving the naming collision, you fixed the problem because the name is now unambiguous.

These problems can happen very easily when you’re using DHCP and sharing a network and domain name between your clients and upstreams, so I think using home.arpa for one and your other domain for the other was a good idea.