Some more info:
The post at 4.2 equivilant for 4.1 networking commands? mentions that rules should be added to custom-forward. The iptables interface adds them to ip filter. Not sure if this is a bug because I don’t see what the source for that claim is (neither of the linked bugs mention it). However, I tried adding the rules to this chain with nft add rule ip qubes custom-forward ip saddr $SENDING_IP counter. This rule is broader than the the iptables rule I was using (no destination address is specified), but that shouldn’t prevent my target qube from receiving the data. After adding this rule (and deleting the rules from ip filter) the behavior does not change (tcpdump shows the packets in sys-firewall but not the target qube) and the counter for the new rule increments.
Additionally, I monitored the interface on the target qube in wireshark and I saw 2 ICMPv6 router solicitation packets sent to the target qube. One came from the sending qube and the other from an address I do not recognize. Based on this, I repeated the nft add rule ... from above with ip6 instead of ip, but this had no effect on behavior. The counter for the ip6 rule did not increment.
Finally, I tried pinging the target qube’s IPv6 address which led to different results. The sending qube sent several Link-Local Multicast Name Resolution requests but did not receive any replies. The echo requests never appeared in wireshark.