Then it seems unrelated to the router's IPv6 Router Advertisement (RA).
Since this issue appears to be specific to the desktop Linux environment, we recommend seeking assistance from the Ubuntu community.
As our team's expertise is focused on network equipment, we may not be able to provide the best advice for this particular situation.
Alternatively, try using a fixed random IPv6 address which hides the MAC address from the network and should not have lifetime issues.
network:
version: 2
ethernets:
eth0:
# Other configurations...
ipv6-address-generation: stable-privacy # add this line