The glitch where an IPv6 address appeared on WAN2 even though IPv6 was disabled on that interface hasn’t happened again after I simply hit Save & Apply with no changes.
I’ve now booted into version 23.09.1. From what I can see, the key difference in the GUI is how interface status is reported: in 23.09.1 the interface stays down until the PPPoE session is actually established by mpd5, whereas in 23.05 the interface flips to UP as soon as the virtual pppoe0 device gets the UP flag—even though the PPP session hasn’t finished negotiating. I don’t think this behaviour affects anything functional, but it is worth noting.
At this point my working theory is that the problem only occurs when both WAN links use DHCPv6. If I switch WAN2 to SLAAC (which is also an option), the disconnect/connect cycle works fine and I don’t see any issues.