First thing I do on a fresh Linux install is set ipv6 to deactivated. Fixes all my initial Linux install problems. I don't question it, it just works every time.
It's a standard Asus router but it's given me a lot of ire. I hate to say it but it's never a problem when I install windows on the same machines
(I'm currently in the process of trying to completely remove windows from my life)
IIRC, a workaround was to prevent Linux from setting this field, or force-reset it on every outbound packet using netfilter.
It's usually bad configuration done by the router vendors. It doesn't mean IPv6 is bad.