I am guessing that this may have something to do with a limitation of Boulder/the Let's Encrypt resolver to only handle 512-byte responses.
I think this has been patched in master but I am not sure if it has made it to production.
For example, doing CAA or AAAA queries to your resolver results in a response over that size:
;; Received 541 bytes from 82.197.194.135#53(ns1.slxh.eu) in 386 ms
edit: Actually I'm no longer sure at all that this is the problem :\