Did you add additional NS records for the forum.omaggieconcorsi.online subdomain, or is this a quirk of the DigitalOcean DNS interface?
To be clear, you only need NS records for your root domain (omaggieconcorsi.online) unless you plan to delegate DNS to different name servers for specific subdomains.
If you added them yourself I would remove them again, it will work without.
Also please note that while you have redirected three domains to your droplet, Discourse can only operate under a single DNS address. The others will redirect as long as the protocol isn’t specified.