Hi!
Use non-local gateway through interface specific route, this option enabled and I get only 2 warnings during bootup and that`s it, no more.
What I think is happening:
I use pppoe ipv6 and when device boots it sets default gateway to fe80::2c1:64ff:fed8:3f00 but errors, because pppoe is not yet fully initialised (first ipv4 and then ipv6).
Also fe80::2c1:64ff:fed8:3f00 is not on the same subnet as IPv6 address on pppoe iface so that is why I was getting errors, and use non local gateway checked shut it up.