CR#6535669 changes tunnels static assumptions about the encapsulating packet's header size to a dynamic check when determining how much to subtract from an ICMP need to fragment's mtu for tunnel overhead.
IPv6 encapsulation can have an encapsulation limit option which was already being accounted for statically and is now accounted for dynamically as well so it is now accounted for twice.
Work Around
This may lower MTU toward the minimum IPv6 mtu (1292) for those using encap limits but should have no impact other than the result of using smaller packets on performance. Alternatively, one could avoid the encaplimit option on their tunnels but not get their benefit (See ifconfig(1M) encaplimit/-encaplimit).