Is it possible that the published app is not signed by its developer andd if yes, How can we find out who is the app developer?
Yes it's possible, you can check by unzipping the APK, finding the
META-INF/CERT.RSA file which contains the developer's public key and using the command
openssl pkcs7 -in /path/to/extracted/apk/META-INF/CERT.RSA -inform DER -print.
For more info have a look at the first section of this link: https://www.evilsocket.net/2017/04/27/Android-Applications-Reversing-101/.