Almost every ad platform is moving to have their ad DNS server names into the same mix as content servers. Without packet sniffing they are practically indeterminate.

Current list off the top of my head: YouTube, Netflix, Peacock, Disney, Paramount+.

It's more costly for them, but 🤷

You have an outdated app that isn't aware of that. Keep it until they force you to upgrade.

Google devices (so maybe apple ones as well) sometimes hard code the dns server so you need to redirect dns request to you local unbound instance with a firewall rule.

On Android you have to disable the use of Private DNS (or something like that) in the WiFi settings. If this is enabled it will not use your WiFi's broadcast DNS, but a predefined one. The reasoning behind this (you can believe this or not) is so that on public WiFis your DNS queries cannot be tracked or manipulated by setting up a local DNS.

I can imagine there is something similar for iOS.

If this only affects some apps then they have decided to hardcore their DNS servers. The only thing you can do now is to identify these and block the calls, but this may also break other stuff if the servers are not only used to resolve ad URLs.