Fixing nginx sub_filter? - nginx-reverse-proxy

Question

I'm trying to fix a misbehaving app behind my reverse proxy - basically, it drops absolute URLs into pages.
My conf looks like
...
location /openproject/ {
proxy_set_header Host <internal host>;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_pass http://<internal host>/openproject/;
sub_filter 'http:<internal url>' 'https:<external url>';
sub_filter_once on;
sub_filter_last_modified on;
}
...
things in <> are replaced to protect my hosts.. and yes the misbehaving app is OpenProject
It doesn't seem to be working and debug doesn't say anything about substitutions happening..
Is there anything that I've done wrong?
nginx -V is
nginx version: nginx/1.18.0 built with OpenSSL 1.1.1k 25 Mar 2021
(running with OpenSSL 1.1.1o 3 May 2022) TLS SNI support enabled
configure arguments: --with-cc-opt='-g -O2
-ffile-prefix-map=/build/nginx-q9LD4J/nginx-1.18.0=. -fstack-protector-strong -Wformat -Werror=format-security -fPIC -Wdate-time -D_FORTIFY_SOURCE=2' --with-ld-opt='-Wl,-z,relro -Wl,-z,now -fPIC' --prefix=/usr/share/nginx --conf-path=/etc/nginx/nginx.conf --http-log-path=/var/log/nginx/access.log --error-log-path=/var/log/nginx/error.log --lock-path=/var/lock/nginx.lock --pid-path=/run/nginx.pid --modules-path=/usr/lib/nginx/modules --http-client-body-temp-path=/var/lib/nginx/body --http-fastcgi-temp-path=/var/lib/nginx/fastcgi --http-proxy-temp-path=/var/lib/nginx/proxy --http-scgi-temp-path=/var/lib/nginx/scgi --http-uwsgi-temp-path=/var/lib/nginx/uwsgi --with-compat --with-debug --with-pcre-jit --with-http_ssl_module --with-http_stub_status_module --with-http_realip_module --with-http_auth_request_module --with-http_v2_module --with-http_dav_module --with-http_slice_module --with-threads --with-http_addition_module --with-http_gunzip_module --with-http_gzip_static_module --with-http_sub_module

Answer 1

Found this https://marsbard.github.io/2016-07-30-replace-urls-behind-nginx-reverse-proxy/
seems I needed the config to be:
location /openproject/ {
proxy_set_header Accept-Encoding ""; # no compression allowed or next won't work
sub_filter "http://<internal host>/" "https://<external host>/";
sub_filter_once off;
proxy_set_header Host <internal host>;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Host $host:$server_port;
proxy_pass http://<internal host>/openproject/;
}