I got curious about what "passthrough" might be doing and found this assertion [0], which reminded me of the existence of '6relayd' [1]. So I assume that that mode relays the RAs & etc, but replaces the link-local address in the RA & etc with that of the relaying interface.

[0] <https://www.snbforums.com/threads/ipv6-passthrough-disadvant...>

[1] <https://github.com/Yamatohimemiya/6relayd>