Because nginx is outputting home assistant on 443 and you’re pointing 8989 to 443 with your port forward.
You’d have to change nginx to 8989, then change your forwarding from 8989 to 8989, then use 8989 in your url. BTW doing this will cut off access to HA. Then again, i’m not 100% familiar with nginx addon and changing 8989 on it may just change what HA is exposed on. Either way, you can only do one at a time unless you use some DNS service.
Reading that, I also realized that I have NGINX Home Assistant SSL proxy, and I have to switch to Nginx Proxy Manager. Do you agree? If that’s your case, would you mind sharing your config structure for the multiple domains with NGINX?
PS: I assume I have to create additional subdomains in duckdns too, right?