I'm seeing an interesting behavior on my RB751G running 5.18. I have my box acting as a home router connected to a cable modem with an ISP doing DHCPv6-PD. While I can sometimes get a prefix into a pool, most of the time the ipv6 dhcp-client errors after a while with no prefix allocated.
When I do a packet capture of the outward facing interface, I get an interesting result. I see the DHCPv6 Solicit message go out and I receive two different prefix advertisements from 2 different server ids. The dhcp-client sends back a request packet with the prefix from the first advertisement, but uses the server id of the second advertisement, so the second dhcp server sends back a negative reply.
I've attached a text capture file showing what I'm seeing. There's two attempts in it, one with the first dhcp server responding first, the other has the second server responding first.