Hey @blueCoconut94!
I’ll preface this by saying that NGINX on Windows is considered an experimental build and we advise against using it in production (and do not support it either). That being said, if you go to https://mytestserver.domain.fr/ords/r/app_dev/ directly do you see the site you expect? Did you share your entire NGINX config (nginx -T)? Can you paste the output of your access log?
I am actively trying to reproduce a similar environment and everything seems to be working as expected on my end.