I think this is working as intended. When the DNS server doesn't respond, or returns an error in the default mode we fallback to local DNS and the page loads fine, but you're not getting DOH protection. If you set it to increased protection then you'll get errors when that remote DOH server is busy or rate-limits you.
Bug 1837717 Comment 2 Edit History
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I think this is working as intended. When the DNS server doesn't respond or returns an error, in the default mode we fallback to local DNS. The page loads fine as far as the user can tell, but there's no way to know when you are or aren't getting DOH protection. If you set it to increased protection then we won't load content using insecure DNS, so when secure DOH requests fail you won't get your content when the remote DOH server is busy or rate-limits you.