Chances are it’s because your server isn’t presenting the intermediate CA certificate chain. Otherwise, you’d have to ask Patreon about that, it’s a problem being reported by their systems, not Discourse. It’s unlikely that Digicert’s root CA isn’t trusted by them, it’s a fairly common and well-regarded trust anchor.
Created the certificate again this time making sure to add the intermediate certificate.
Created a new load balancer.
Added the new certificate to the load balancer.
Added the old droplet to the new load balancer.
As far as I can tell, you can’t update either the load balancer or the certificate so deletion/creation is required.
Thank you so much for the speedy and helpful response. I never contacted Patreon but something tells me I’d be unlikely to receive this level of support over there.