I can't submit a reply through feedback app since I am not a developer.
But if I have WhatsApp open and another app open then double tap the home button. It frequently freezes up or I am unable to swipe around.
You don't need a developer's account to report bugs to Apple via:
http://http://bugreporter.apple.com/
And Apple doesn't care about 3rd party apps that don't work with beta releases of their operating systems. You might like to let the 3rd party app developers know, just as a courtesy.