Unless I’m misunderstanding, that doesn’t sound like a bug at all. Outside of a few specific circumstances, devices shouldn’t communicate with anything outside of the given subnet mask. Rejecting traffic outside of that subnet mask is exactly what it should do. And why wouldn’t your pihole be in the same subnet (or at least be included in the subnet mask) for the LAN? You can have the pihole’s IP address be whatever you want, so give it an IP in the same subnet.