Ipv6 sometimes doesn't connect on boot

This wouldn't be a problem usually but both ipv4 "wan" and ipv6 "wan6" are bound to eth0

so LEDE detects eth0 is up when ipv4 is up and doesn't try to bring up wan6 because eth0 is up already.

when i manually reconnect wan6 it's working fine afterwards.

Any way I can split up wan/wan6 from eth0 to eth0.1 and eth0.2?

or, how to tell lede to detect wan6 is down and needs reconnect even if wan is up?

You might start with the Basic configuration section in the LEDE User Guide...