>>108620154
Your router might not be sending router advertisements quickly enough for it to enable it.
On my OpenWRT router I ended up needing to configure Uradvd (I'm not using Odhcpd or Dnsmasq because I wanted a simpler config) like this:
option max_router_adv_interval '10'
option min_router_adv_interval '3'
option default_lifetime '1800'
Without that Android devices wouldn't connect to the IPv6 mostly network quickly enough and would display connection error messages. Maybe Windows is similarly picky? Linux on the other hand it doesn't care, it see's IPv6 10 minutes later than it should be (maybe that's how long your shit system took to boot anyway) and then configures it.