Domain on allow-list still being blocked

I run my own Mastodon instance.  I run the software, and the server, and I run my own signed DNS zone, too.

When I'm using my instance on the web (https//mastodon.mydomain.com), I would often get a full-screen message saying "mastodon.mydomain.com is not blocked anymore :-)" which would persist for several days. This was only on my desktop, which uses NextDNS via Firefox's DoH settings. My phone (also using DoH) never got this error and everything was fine.

So, I added mydomain.com to my allow-list. The browser eventually started loading, but random things fail. I checked the logs and see random sub-resources failing with a header Blocked-By: NextDNS. Again, my phone is just fine. I checked my NextDNS logs and nothing is being blocked; it says all DNS A/AAAA/HTTPS lookups to mastodon.mydomain.com are going through.

I don't know what's going on, and I don't know how to contact support.  As a paying customer, I'm quite upset and I'd like to learn how to fix this.