I've never used Nginx proxy manager but would expect that it would "just work" as confirmed working config (see below) is fairly minimalist vanilla Nginx reverse proxy config - without anything special.

Regardless, if you could actually share the Nginx config that it has generated for you, perhaps I can see what has gone wrong (and you can report upstream to Nginx proxy manager devs)?

FYI, to test, I launched a vanilla v17.1 WordPress appliance and a v17.1 Nginx appliance. Other than running through the firstboot scripts, I changed nothing on WP.

On Nginx, I disabled the 'tkl-default' site ('rm /etc/nginx/sites-enabled/tkl-default') and created a new config file (/etc/nginx/sites-available/wp-rev-proxy) as below (note this doesn't support IPv6 - let me know if you need IPv6 and I can include that too):

server {
    listen 80 default_server;

    # SSL configuration
    listen 443 ssl default_server;
    include snippets/ssl.conf;
    server_name wp.jeremydavis.org; # change this to your domain

    add_header Cache-Control public;

    location / {
        proxy_pass https://192.168.1.121; # change this to your wp server
        proxy_ssl_verify off;
        proxy_redirect off;

        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $remote_addr;
        proxy_set_header Host $host;
    }
}

After adding that, I enabled it ('ln -s /etc/nginx/sites-available/wp-rev-proxy /etc/nginx/sites-enabled/wp-rev-proxy'), restarted Nginx ('systemctl restart nginx') and it "just works" for me!?

Note that whilst the config above works, it could be improved (it's pretty basic and minimalist).

I can only assume that Nginx proxy manager is doing something weird? I'm assuming that there must be some way to match the options in my config? And/or perhaps you can do some sort of site config manual override?

Good luck and please let me know how you go.