Temporary IPv6 lifetime is reset by router advertisement

Hi there,

I observed an odd behaviour on my laptop regarding the privacy extensions for IPv6. I correctly get a global dynamic IPv6 with lifetime 14400. However every few seconds the counter on the lifetime is reset to 14400, basically converting the temporary address into a fixed one.
I started a tcpdump and could observe, that the reset happens when a router advertisement for my prefix is received:

12:27:12.631357 IP6 (hlim 255, next-header ICMPv6 (58) payload length: 120) fe80::1:1 > ff02::1: [icmp6 sum ok] ICMP6, router advertisement, length 120
 hop limit 64, Flags [other stateful], pref medium, router lifetime 30s, reachable time 0s, retrans time 0s
   prefix info option (3), length 32 (4): a:b:c:d::/64, Flags [onlink, auto, router], valid time 86400s, pref. time 14400s
...

This has the 14400 seconds in there as well. Hence I assume that the router advertisement is resetting the ip configuration.

This feels like a bug, as it basically converts the temporary address into a permanent one as long as the laptop is online?
The lifetime should count down and then a new IP should be created.

I am running Linux Mint 19 (Ubuntu 18.04)