Try this way:
ip -6 route get 1::
ip -6 route get 1:: dev pppoe-wan
ip -6 route get 1:: dev pppoe-wan from 2804:1b2:206:520:a9f2:ca03:4dec:7c71
ip -6 route get 1:: dev eth0.3
ip -6 route get 1:: dev eth0.3 from 2804:14c:658b:1000:45d7:3d14:cfdc:66cd
ping6 -c 3 example.org
ping6 -c 3 -I 2804:1b2:206:520:a9f2:ca03:4dec:7c71 example.org
ping6 -c 3 -I 2804:14c:658b:1000:45d7:3d14:cfdc:66cd example.org
It seems that in some scenarios OpenWrt can't correctly select the source IPv6 address for routing.
I have a similar bug report here:
https://bugs.openwrt.org/index.php?do=details&task_id=2167
The only workaround is to add another IPv6 default route without source address filter:
https://openwrt.org/docs/guide-user/network/ipv6/ipv6_henet#default_route