From the link in my last post of why Let's Encrypt uses this setting,
"Authoritative resolvers that return large responses but don’t support TCP would stop working."
I think this describes what's going on for your case.
Taking a quick look at the page you linked of the DNS server you used, there's an issue for improving their documentation of how to run a server that does both UDP & TCP.
So you might want to see if you can configure it to serve on TCP as well as UDP, and you're much less likely for systems to have trouble resolving your name. (Or you may want to switch DNS software entirely, to something more popular for authoritative DNS serving, though I certainly understand that might not be what you're looking to do at this time.)