NextDNS blocking YNAB app on iOS when on cellular network

I found a strange problem. I'm running NextDNS on an iPhone 12 with iOS 14.7.1 and I'm also using the YNAB (https://www.youneedabudget.com/) app.

They have a help section integrated in their app and whenever I'm on the cellular network and NextDNS is active, the help section does not work ("no internet connection"). There are no blocked connections visible in the NextDNS logs. 

It all does work when I disable NextDNS completely. Also this problem does not occur when I'm on WiFi. 

Anybody has an idea where the problem is? iOS Bug? NextDNS bug? 

Thanks