There is a great deal more going on in your web server than it seemed from your initial query. In particular, the actions of the proxy_* directives might be confusing things almightily. There may be reasons for this complexity, but they are beyond my pay grade. I recommend that you turn on some debugging in the server configuration, and then use a browser to access the .well-known/acme-challenge URL we have discussed above. That will give you some information about what actually happens to this query, and then may point toward a resolution.
Sorry I can’t help more from this distance.