_acme-challenge cname (as per acme-dns) broken

I contacted my provider and they confirmed that the issue is with their nameserver not delegating the acme-dns zone correctly, and promised to fix it.
While debugging this problem I found two tools I will keep handy in the future for checking delegation issues like this:
https://simpledns.plus/lookup-dg
https://www.denic.de/en/service/tools/nast/

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I think that if each zone (specifically the one containing _acme-challenge entry) were to explicitly define the name servers used, one could walk from "." to your txt entry (without fail).
[and should be an easy test to conduct]