21.02.0-rc3 ipv6 client cannot got IP

I have two Routers:
Router1 (19.07.3)
Router2 (21.02.0-rc3)

I created a vlan interface at each router, with IPv6 only - the address is being auto assigned from the DHCP-PD of the WAN6 interface.

When I connect Fedora 34 clients to that vlan of routers one by one:

  • w/ Router1, client can get IPv6 address
  • w/ Router2, client cannnot got IPv6 address.

There are lots of new options in the DHCPv6 settings in 21.02.0.

How to make it working like a 19.07 DHCPv6?