I am seeing the subdomain wiki pointing it’s A record to IP 59.167.127.21, that is not a MX record entry…
No, your DNS is handled by the box, so all entries are either handled by the MiaB management daemon itself, or by editing Custom DNS.
Now this is what I am seeing at this moment. Your domain’s glue records are set. WHOIS is showing your MiaB to be your name servers. Your name servers are serving your domain correctly, however I am not seeing an entry for the subdomain wiki.
Where are you planning to host the subdomain wiki? On another server, or on the same server as MiaB?
Stop the presses: Remove your DNSSEC entry from your domain registrar. Your zone records are not being propagated because DNSSEC is failing. Remove it completely. Once things are working properly you can add it back on (using the correct keys) again if you wish to have your domain use DNSSEC.