My ISP serves IPv6 in a way that prefix delegation is detected by OpenWRT correctly, but it doesn't assign router itself an IPv6 global address. Something similar was happening for example with OpnSense and they worked around that somehow very recently. DD-WRT assigns it too.
Is there some setting in OpenWRT for that?
Hm, I think I figured it out, I was looking at the wrong place (WAN interface). br-lan one shows IPv6 address which is address of the router.
You should have an IP on WAN if you have one on LAN. Maybe you edited the wan6 interface?
Nope, it's default. And WAN doesn't show anything besides the local link address. I.e. no global scope.