Comment by bigfatkitten

NPTv6 is rarely used, and so its real world implementations tend to be poorly tested and buggy.

The answer in this case ends up being solutions like explicit web proxies, or alternatively a VPN concentrator or the like from which you can receive a routable prefix delegation, and then run multiple tunnels to satisfy your own availability or policy routing needs. Either way, you’re building some complex infrastructure to overcome regressions imposed upon you at layer 3.