Linksys WRT1900ACS v2 - v21.02 - DHCPv6 on LAN interface

After upgrading from v19.07 to v21.02 on a Linksys WRT1900ACS v2, I reimplemented the configuration, although I can't make IPv6 working.

One problem I spotted is my PC is not using the proper IPv6 link-local address as gateway.
Here is the IPv6 address from the router:

2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1508 qdisc mq state UP qlen 1024
    link/ether c4:41:1e:30:40:ff brd ff:ff:ff:ff:ff:ff
    inet6 fe80::c641:1eff:fe30:40ff/64 scope link
       valid_lft forever preferred_lft forever

My PC get allocated a publicly routable IPv6 through DHCPv6 on its interface talking with the router, which I can ping from the router, but its associated gateway is wrong: fe80::c641:1eff:fe30:4000
The route table confirms that:

::/0                     fe80::c641:1eff:fe30:4000

Why are the last 8 bits from the link-local IPv6 from the router interface zeroed in the routing table of the PC? It looks like an odd /120 representation of the original address.

I rolled back to v19.07: no working IPv6 is a game changer.
That, added to the other problem I encountered.

Opened a GitHub issue openwrt/openwrt#9677 related to this.