DNS lookups failing (iOS)

Hi, I'm trying to track down a strange problem with a business application, and I think I've found a similar issue (perhaps with the same root cause) with various network test tools, including the one that I'm currently using - NSLookup by Dayana Networks.

The behaviour looks like this:

Using the app do to test DNS lookups, when my iPhone is using my home router as the DNS server fail (time out), but other all other apps such as Chrome/Edge/Safari/Firefox/Outlook/Teams/YouTube etc etc are all ok.

If I set up a simple DNS server on my home network, and either point the app or the iPhone to use its IP as a manually-set DNS server it also fails - no DNS request is received or logged by the server at all.

If I use 8.8.8.8 (for example) for the manual/app's DNS server, resolution works fine.

And here's the weird bit - if I set up my router to forward port 53 from the internet to my simple DNS server, and use my public IP as the DNS server on the phone/app, DNS resolution works fine and I can see requests coming in and answers being returned.

Using mobile broadband is a similar issue - using Vodafone's (my mobile provider) default DNS server fails, but using 8.8.8.8 works fine.

I asked a colleague to check his personal iPhone and he found that DNS lookups using vodafone's default resolver failed as well, but when he switched his SIM to a different provider (Three) name resolution worked perfectly.

I've run some packet captures over USB to a macbook, and when I'm using a DNS server on my home wifi network it's like DNS requests aren't being sent by the iPhone at all. General traffic is being logged, but nothing that corresponds to the DNS requests being sent by the app.

I'm wondering whether it's because the NSLookup app is using one of the non-recommended resolution methods documented here: Resolving DNS Hostnames

Perhaps the latest apps use the asynchronous method, and the NSLookup app is using either the posix one or the non-asynchronous methods?

I'm at a bit of a loss, and hoping someone will be able to shed some light on this!

Best wishes,

Iain