An update to this. After upgrading to routerOS 7.6 and reconfiguring everything I was still running into problems.
Then I decided to dig a little deeper and perform some basic debugging directly on the mikrotik - no problems there could ping and traceroute to addresses like 2600:: just fine.
Then tried the same on my PC - all good. Then went back to
https://test-ipv6.com/ and it gave the same errors.
I then tried
https://ipv6-test.com/ and it is fine. As is googles more simple website to test IPv6.
SO it looks like there may not be a problem with my installation at all or Afrihost but rather the silly
https://test-ipv6.com/ website.
So this brought me back to the reason I opened this can of worms in the first place. I had noticed that my sonarr installation was no longer working with nzbfinder.ws as an indexer if I had IPv6 enabled. Investigating further I see that the name server is returning an ip address of
2001:41d0:0306:119d:0000:0000:0000:0000 for nzbfinder.ws - that looks completely wrong to me but hoping to ask the experts here. Could me original issue just be an incorrectly configured AAAA record for nzbfinder.ws that is outside of my (and afrihost's) control?
Or is anyone else able to ping that IP address?