iOS 26 / 26.0.1 - No or slow DNS on cellular

Hello,

after I installed iOS 26 on my iPhone 14 Pro, DNS requests for installed NextDNS app in mobile became extremely slow or broke off with timeout. Uninstalling of the app and setting up a NextDNS profile also did not solve the problem.
There were no problems in the WLAN.
Also, a reset of the network settings and a restart of the mobile phone did not bring a solution.

Even after switching to an iPhone 17 Pro, the problem was reproducible. With other DNS profiles, e.g. Mullvad, it works without problems.
Even use without private relay has not brought any improvement.
iOS 26.0.1 also not.

I tested all this in the network of Deutsche Telekom and Vodafone. To my knowledge, both networks rely entirely on IPv6.
My landline is also natively IPv6, but the problem does not occur in the WLAN via it.

The issue does not occur on an iPhone 13 Pro with iOS 18.x.

Is anything known that Apple has changed anything in the current versions that can trigger this behavior? And if so, how can I solve the problem?

Greeting
Wutti