Strapi v4 Heroku deployment error R10 failed to bind to $PORT within 60 seconds of launch

I’m facing the same issue. Once the app worked all of a sudden after a couple of hours or two. But then now i’m stuck! everytime i create a new app it works fine and thats it! when I push a change then I get this error. I’m using the free dynos and i’m not sure that’s causing the issue. Anybody here using paid dynos facing teh same issue?