Just realised that the phone is using the Pi as a DNS server, or at least it seems like it is...
That second IPv6 address is probably not Pi-hole ?
Problem still occurs, even after a reboot 
Ah-hah! You're probably right! %$#@ing IPv6 screws with me yet again...I've discussed this with others in my first thread. Don't suppose you know anything about disabling IPv6 on my LAN interface in OpenWRT?? And how would I disable it on the Pi?
Hold on.
LOL thanks mate. Appreciate your time and effort put into this 
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Yeah I vaguely remember running those commands before, as suggested here. My Google search for "openwrt disable ipv6" brought that up as the first result, and showed it as a previously-visited link. So I think that's how I "disabled IPv6" to begin with...or at least I thought I had!
EDIT: Just remembered what the problem probably is: I reflashed OpenWRT on my router recently, since I was having trouble with some other settings that I had misguidedly enabled. But re-running those commands still doesn't seem to have changed anything...my phone still shows that IPv6 address as a DNS server :@
Having said that, check this out:
C:\Users\username>nslookup fd77:bff1:b4d5::1
Server: RaspberryPi
Address: 192.168.1.100
*** RaspberryPi can't find fd77:bff1:b4d5::1: Non-existent domain
Doesn't that last line mean that it's not a valid IP address?
Switching off IPv6 on your router will just stop it from distributing a global public IPv6 prefix as supplied by your ISP.
It wouldn't stop your devices from assigning at least link-local IPv6 addresses for themselves, or a ULA address if they can get hold of a ULA prefix somehow. You'd have to switch off IPv6 on every single device individually to be sure to cut IPv6 from your network.
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Thanks, I'll give that a go. Wish me luck!
Thats just IPv6 addresses for the devices and not the IPv6 DNS servers assigned/discovered am I right ?
According to the first answer on the SuperUser thread that I linked to above, it should do just that! Or am I misreading it?
Hmm I was afraid that might be the only way...I guess I'll have to consult The Oracle some more and find out how to do that on Android. Thanks for the tip 
With above one, your querying the OS configured DNS server 192.168.1.100 to lookup the domain "fd77:bff1:b4d5::1".
Syntax is:
nslookup <DOMAIN> <DNS_SERVER_IP>
LOL oops! How about this instead:
C:\Users\username>nslookup google.com fd77:bff1:b4d5::1
Server: UnKnown
Address: fd77:bff1:b4d5::1
*** UnKnown can't find google.com: No response from server
I've now tried these instructions to disable IPv6 on the phone, as well as setting a static IP and DNS settings for my WiFi connection, but Android still seems hell bent on using the DHCP-provided settings. This is really getting beyond a joke now...
If the Windows box isnt configured for IPv6 (from ipconfig /all), it wont be able to lookup through IPv6 DNS servers.
C:\>nslookup google.com fd77:bff1:b4d5::1
Server: UnKnown
Address: fd77:bff1:b4d5::1
*** UnKnown can't find google.com: No response from server
If some IPv6 RA are offered or a DHCPv6 server is found, devices may still try to assign a DNS server via IPv6 as well, but without a global prefix, they won't be able to reach out to the public Internet.
It would depend on the device whether it would try to assign IPv6 addresses at all in absence of both, so to be sure you really dump IPv6, you'd have to switch off each device.
But as far as IPv6 bypasses to public DNS servers are concerned, you're safe if no global prefix is offered.
Yeah well I thought that by disabling IPv6 on the router, I would be ensuring that no global prefixes would be offered. But I guess I was wrong:
Ahh good point! Didn't think of that! D'oh!
Trying the same command on my phone indeed resolves google.com properly...so I guess that IPv6 DNS server is definitely the cause of the issue. Now if only I could find a way to reliably turn the @#$%ing thing off...
Anyway thanks everyone for your time and responses. I'll do some more digging and figure out how to set static DNS settings on my phone, and actually make them stick.
Dont run Android updates yet if want to continue using the "Terminal Emulator" app for diagnosing!
Mine was updated couple days ago.
I wanted to try out something and when opening the app, I got a warning with something like its not compatible anymore.
None of the commands entered work anymore.