I must be going mad. When I said 40 bytes back their I should have said 20. My apologies
An IPv4 proto 41 tunnel, used to encapsulate IPv6, adds an IPv4 header to the start of an IPv6 packet. The IPv4 header is 20 bytes, so the tunnel payload, the IPv6 PDU, will have an MTU that is reduced by 20 bytes compared with the normal IPv6 PDU MTU.
So given my stupidity, I don’t have an explanation for the numbers you’re getting. You do indeed add on 28, as you said,