@Jeelani my suggestion would be to disable PreValidateDns in settings.json as per my link. The particular method it's using (looking at your system DNS) is not ideal and instead it should be querying your domains primary name servers for the definitive answer (if at all), but it doesn't do that.
unboundtest can clearly see your TXT record, so everything else public should as well. It's possible/likely that your internal DNS does not replicate public DNS settings and that they are two different zones for the same domain (one public, one internal).
https://unboundtest.com/m/TXT/_acme-challenge.khaneducation.net/3HW3623D