Actually, yes. There's another thread: https://support.mozilla.org/en-US/questions/1579171 (and one older https://support.mozilla.org/en-US/questions/1578799 with some AI slop answer, which however doesn't sound completely off…) in case you're also running in "maximum protection" DoH setup.

I don't see an open bug filed, maybe it's worth making one?

Thank you. The slop answer seems pretty good, i filed an bug (https://bugzilla.mozilla.org/show_bug.cgi?id=2037364)

Thanks, this really helps. I've seen some similar reports in feedback channels, but nobody actually filed a bug so this now gets in front of the networking folks to make sure it was indeed intended this way.

(Read: people have been surprised by the opposite: having DoH fall back to something else on that setting, esp. for DHCP "local" search prefixes like captive portals and other configurations, so it's better it errors out loudly instead of a fallback — however I can imagine the UX for that be at least aware of the feature clash, and hint at relaxing one of the sides, or for the features to allow poking holes in the setup to work for such combinations. I've linked several other reports to your bug — thanks again for taking the time to file one to be able to track the progress yourself and get in touch with the engineers if any questions arise. 💪)