That's why I specified if one was using a static v6 network. There are several reasons why this might not be true, from ipv6 CGNAT like what cell providers do, to ISP rotation, to randomization in your own network, to NATing from the private network if you wanted.

But it does seem like it would be far more likely de facto for an ISP to not randomly rotate v6 networks, except maybe to discourage hosting on consumer connections?