"Empty" configuration breaks Android app?

So I have this recipe-app on my Android, "Cookiedoo". Turns out that out of the hundreds of apps on my phone, this one seems to take badly to me switching to my NextDNS PrivateDNS setting on Android 11. Once I enabled PrivateDNS, the app's homescreen (probably some embedded Web view, looking very much like, e.g., https://cookidoo.co.uk/) times out (all I see is a spinning circle, see screenshot). It finally claims that I'm not connected to the Internet (screenshot 3). Remove PrivateDNS - and all works in less than a second (see screenshot 2).

After lots of fiddling around trying to isolate a potentially problematic tracker that was blocked and then whitelisting it (none of which worked), I ended up with the following experiment: I created a new NextDNS configuration and switched _off_ all filter lists and protection etc. No privacy, no security. Nothing enabled. I then switched my Android's PrivateDNS to this "empty" configuration. And started the "Cookiedoo" app. 

The same timeout behavior :-O 

So I start to wonder: what is it that breaks this app? What does it do that suddenly doesn't work when I enable PrivateDNS, even with an "empty" configuration (i.e., nothing is blocked)? 

To add more mystery: Turns out that I can "unbreak" the app by doing a task switch: When I switch to another app on my phone while waiting for the timeout, but then quickly switch back, the app will have actually loaded the Web view "in my absence"! It will only timeout if I "watch" while it tries to fill the Web view.

Last but not least: once the Web view has been loaded once, I can then freely navigate through the app (e.g., change screens), everything works for a while. So clearly some caching involved here. Hence I can always work around the problem by disabling PrivateDNS, start the app, wait for the initial Web view to load, then re-enable PrivateDNS. Or do this task-switcheroo. Both bit of a pain though.

Any thoughts? Happy to do screen recordings to make this more clear...