According to https://wiki.mikrotik.com/wiki/Manual:IP/DNS when using DNS entries, e.g. forwarding of DNS using regex one should use lowercase characters as mikrotik converts uppercase to lowercase automatically.
However, with 7.1b6 this does not seem to work. If I do “nslookup -type=srv _kerberos._tcp.AD.DOMAIN.COM” and only match a regex against “.ad.domain.com” then the name cannot be resolved. Interestingly, if I first check “nslookup -type=srv _kerberos._tcp.ad.domain.com” and then use “nslookup -type=srv _kerberos._tcp.AD.DOMAIN.COM” it works.
My guess is that the internal cache is used then.
Also if the FWD DNS is not reachable, the normal DNS is used and cached which can result in the wrong IP being stored in the cache, if the forwarded DNS is temporarily unavailable.