Let us know, 12 bytes going to thin air isn’t possible. Someone suggested it’s “padding” exclusive to MikroTik which happens to be undocumented.
Perhaps you can help this gentleman realise my solution to broken MTU is the most effective and works for UDP, not just TCP via MSS clamping: http://forum.mikrotik.com/t/some-websites-unavailable-on-ipv6/145044/13