There was a bug in the luci app where the bootstrap_dns servers from the providers' json files. Sadly I can't find neither the issue created for it nor the forum post, so whoever discovered it and alerted me to this issue, please make yourself known.
The fix I implemented might have side-effects, but I'm very short on time, so I haven't tested all the possible use cases.
I'd appreciate as many people as possible testing luci-app-https-dns-proxy 2025.05.11-r2 from my repo (you do NOT need to update the principal package, but it got a version bump to stay in sync), doing as much thru WebUI as you can.