What you're observing might be your operating system's behavior. There is a privacy feature of IPv6 called "privacy addresses" (or sometimes "temporary addresses").

https://tools.ietf.org/html/rfc4941

Some (most?) operating systems rotate the v6 privacy address daily or more often. The benefit of this is a) address not transparently based on permanent ethernet hardware MAC address and b) changes over time. Both are meant to hamper tracking.

If you're on a Mac and using IPv6, you can see these temporary privacy addresses stacking up over time if you type `ifconfig en0`. Old ones don't disappear immediately when rotated out since you need to be able to receive packets for a while. They are marked "deprecated" for some time before they disappear.