After moving my primary device LAN from 192.168.1.0/24 to 192.168.3.0/24, Pihole will be able to read my machine hostnames as it did before the move. Pihole stil resides in the 192.168.1.0/24 network.
Actual Behaviour:
With rare exceptions, device hostnames are not showing up in Pihole Dashboard stats. Instead I get only IP addresses on many previously-known devices after 4 or 5 days since the change. With the new IP addresses, each device shows no MAC address under Tools/Network. I suspect this is why it can't match up to a hostname.
At first I thought it was Apple devices not showing up correctly (privacy settings, etc.), but this is affecting non-Apple devices as well, such as a Windows 10 computer.
I have experimented with conditional forwarding configs but have been unable to affect any change.
Are you using a VLAN controller to pass traffic between the various VLAN's? If so, are you specifically allowing port 53 (DNS) traffic to pass between VLANs?
If you route from your 192.168.3 network to your 192.168.1 network, the only mac address your pihole will see is the mac address of the router delivering packets on behalf of the machines on your 192.168.3 network.
My firewall allows all traffic from any VLAN (I have multiple) to access my Pihole's IP, it then blocks any port 53 or 853 traffic that isn't allowed by the prior rule.
Also DNS lookups are not failing in any way, on any VLAN. This new network is heavily used, and if DNS lookups were failing I would see them immediately.
I just realized my Windows 10 machine is resolving the hostname now. I haven't used the Win10 machine in a few days, so maybe using it now supplied the missing host name. That would indicate only the Apple devices are not showing up now.
So I ran some more tests. I toggled Private Wi-Fi Address on my iPhone off, saw that Pihole was registering the MAC address, then ran a few web page lookups from the phone. I noted the activity in the query log, saw that it showed my iPhone's hostname, and a (different) IP address. However, when I went to Tools/Network and found my iPhone, it showed the correct IP address but N/A for the Hardware Address.
I ran the above test again, got yet another IP address with Private Wi-Fi turned off, and the new IP address showed in Tools/Network but still N/A for the Hardware Address.
So again, it looks like Apple iPhones/iPads are not passing their hardware address info to Pihole even after turning off Private Wi-Fi. This was not always the case; I may have had situations where the hostname was not passing, but I have always gotten a MAC address to show up in Pihole. As a data point, my Apple TV on the same new network passes the hardware address, but it's connected via Ethernet cable and not on the Wifi, so Private Wi-Fi settings wouldn't apply here.
Some change to iOS or maybe even Pihole is at the bottom of this.
As just one data point, I have multiple IOS (latest), MacOS (latest) and TVOS (latest) devices running with multiple versions of Pi-hole with no issues.
I appreciate, as always, the input here! @Moto's feedback (coupled with the data point offered by @jfb) is what made me finally understand what was going on. It didn't make sense at first but after reviewing query logs and recognizing what I had done when moving my devices to a new network, it became clear.
I moved my Raspberry Pi over to the new network, ran pihole -r, and am moving forward. Thanks! My hypothesis regarding the cause of the issue was completely in the wrong universe, never would have figured that out if you guys hadn't chimed in!