Ok I'll check it, but once I've finished talking on Whatsapp I'm always closing the program in the right way, but I don't know at this point if that does matter or not. It could be also Facebook Messenger that is often open and working in the background. In any case I will restart the device for sure I'm gonna keep your app which I love.
It was off! I solved the problem 5 minutes ago removing the contact and customizing it again. Probably something changed after I updated the app, who knows, but thank you once again for your kind reply and your help. :)