nginx proxy subdirectory to subdomain on same machine - nginx

Question

I'm using the docker image from linuxserver called swag which contains an nginx reverse proxy and a Let's encrypt certbot. Quite some dockerized apps are not designed to be accessed via subdirectory proxying but instead need to be proxied to a subdomain (because otherwise js and css files are requested from the domain, not the subdirectory).
My goal is to make a service at 1.test.example.com available at example.com/1
The config for the subdomain looks like this and works fine:
server {
listen 443 ssl http2;
listen [::]:443 ssl http2;
root /config/www;
index index.html index.htm index.php;
server_name 1.test.*;
include /config/nginx/ssl.conf;
client_max_body_size 0;
# some app
location / {
include /config/nginx/proxy.conf;
proxy_pass http://172.2.0.2:1234/;
}
}
My try for proxying to the subdomain looks like this but doesn't work as my browser returns "400 Bad request":
server {
listen 443 ssl http2;
listen [::]:443 ssl http2;
root /config/www;
index index.html index.htm index.php;
server_name _;
include /config/nginx/ssl.conf;
client_max_body_size 0;
# proxy to some app
location /1/ {
include /config/nginx/proxy.conf;
proxy_pass https://1.test.example.com;
proxy_set_header Host 1.test.example.com;
}
}
What is the correct way to do this using proxy_pass and without using rewrite?