adding proxy_buffer_size 128k to Custom Nginx Configuration deletes the configuration

Checklist

• Have you pulled and found the error with jc21/nginx-proxy-manager:latest docker image?
  • Yes - running 2.14.0
• Are you sure you're not using someone else's docker image?
  • Yes - jc21/nginx-proxy-manager:2.14.0 in use
• Have you searched for similar issues (both open and closed)?
  • Yes

Describe the bug
I am adding "proxy_buffer_size 128k;" to the advanced Nginx configuration as I am getting similar issues as described here: https://github.com/NginxProxyManager/nginx-proxy-manager/issues/438
When saving the configuration I get saved successfully but under data/nginx/proxy_host the proxy host conf file is no longer found. Hence when accessing the configured proxy URL the proxied host is no longer available.

Nginx Proxy Manager Version
2.14.0

To Reproduce
Steps to reproduce the behavior:

1. Go to proxy host of choice, in my case host no 7
2. Go to edit
3. Go to advanced
4. Add "proxy_buffer_size 128k;"

5. save the configuration (no error or any indication that something bad happened is shown)

6. go to /nginx/proxy_host to validate the configuration is present and find out no 7.conf file exists anymore:

Expected behavior
7.conf file should be updated with correct config value or an error message should be shown that config could not be saved.

Screenshots

Operating System
Ubuntu 22.04.4

Additional context
none

Reproduce the save flow for proxy host 7 using the nginx-proxy-manager:2.14.0 Docker image and the advanced setting proxy_buffer_size 128k;. Inspect how the configuration is generated under data/nginx/proxy_host and verify that 7.conf is retained with the setting, or that saving reports an error instead of removing it.