Shortly after enabling iBGP between the two DN42 routers I noticed that some of the other hosts inside the DMZ were reporting both routers as default gateways. This was surprising since radvd was disabled but it turned out to be FRR.
I ended up creating a firewall rule to filter out RA packets:
set firewall ipv6 output filter rule 10 action 'drop'
set firewall ipv6 output filter rule 10 outbound-interface name 'eth0'
set firewall ipv6 output filter rule 10 description 'Prevent RA packets being sent'
set firewall ipv6 output filter rule 10 protocol icmpv6
set firewall ipv6 output filter rule 10 icmpv6 type-name nd-router-advert
The issue with FRR sending RA packets in certain BGP configurations is explained here.