Cached route overrides manually-added one?

Hello,

I'm having a strange routing problem, hope someone can help.

Code:

root@rin:~# ip route get 2001:470:aaaa::100
2001:470:aaaa::100 via 2001:470:aaaa::100 dev henet src 2001:470:24:bbb::1 metric 0
cache mtu 1480 advmss 1420 hoplimit 0

Now I need to add a route so that this IP and some subnet including it is accessed not via the default route, but over a VPN interface (and a ULA-addressed router inside the VPN):

Code:

root@rin:~# ip -6 route add 2001:470:aaaa::/52 via fd14:8086:1337::100 dev tun-rm

But that has no effect:

Code:

root@rin:~# ip route get 2001:470:aaaa::100
2001:470:aaaa::100 via 2001:470:aaaa::100 dev henet src 2001:470:24:bbb::1 metric 0
cache mtu 1480 advmss 1420 hoplimit 0

Why it is completely ignored, and the cached route continues to be used?

The complete routing table:

Code:

root@rin:~# ip -6 route
2001:470:24:bbb::/64 dev henet metric 256 expires 21262229sec mtu 1480 advmss 1420 hoplimit 0
2001:470:aaaa::/52 via fd14:8086:1337::100 dev tun-rm metric 1024 expires 21334303sec mtu 1500 advmss 1440 hoplimit 0
fd14:8086:1337::/64 dev tun-rm metric 256 expires 21262224sec mtu 1500 advmss 1420 hoplimit 0
fe80::/64 dev tun-rm metric 256 expires 21262224sec mtu 1500 advmss 1420 hoplimit 0
fe80::/64 dev henet metric 256 expires 21262229sec mtu 1480 advmss 1420 hoplimit 0
default dev henet metric 1024 expires 21262229sec mtu 1480 advmss 1420 hoplimit 0

(why they even have these "expires" values??)

So if I will do "ip -6 route flush cache" at this point, the cached route will go away, and everything will work as I want it to (routing over the VPN).
The problem is, the cached route as shown above will "return" on itself after several days of operation, breaking the routing again in the same manner.
Any idea what is going on here?