[inigyou]

I thought the kernel generated SLAAC addresses based on MAC and privacy addresses based on random numbers.

[tremon]

It does, but DHCPv6 prescribes a persistent device identifier (DUID): https://www.rfc-editor.org/info/rfc9915/#RFC3315-9

The DUID is designed to be unique across all DHCP clients and servers, and stable for any specific client or server. That is, the DUID used by a client or server SHOULD NOT change over time if at all possible; for example, a device's DUID should not change as a result of a change in the device's network hardware or changes to virtual interfaces