Even tho i seem to be talking to myself, i give it a go (again).
Had a 7 day "streak" without any incidents, but the previous night i had a WAN disconnect according to the logs. This in turn lead to me loosing ipv6 a few hours later. Now, i wonder, could it be that whenever i loose WAN connectivity something "hangs" in that RA wont broadcast/refresh my prefix until i either reboot or restart wan? (By saving wan settings and applying without actually changing anything).
It could be just a coincidence that i had 7 days uptime on WAN now vs. before changing nic's, as it usually happen at night (2 pm ish), which COULD indicate some ISP maintenance or something like that. Anyway, the thing is that it does seem as when WAN link goes down, something weird will happen with my prefix.
I just upgraded to 2.2.6 today, but havent really had the chance to study the patchnotes yet, so not sure if this is something that is specifically addressed there tho.
C