Everything seems to be working as it should on both 1.4.3 and 1.5Q2 for stream, I recently started running tailscale in a container which creates tailscale0 and while I was expecting to run into issues with this, I did not, but it got me thinking, does the interface even need to exist to be able to add it to the nftables ruleset?

Yes that appears to fix the issue, I'm seeing complete ARP entries in both tables now.

I've further determined that it also occurs if no container is attached to the network because the interface is only created when a container is attached.