I am using an iPhone 5s with latest iOS update 9.3.5. Whenever I share images using WhatsApp, the app does not respond until the image share/upload is completed. Nothing works, keyboard or back navigation. The app works as usual after image sharing is completed. I have tested this issue with iPhone 6 and 6s too. What might be the issue ? Is this a bug in WhatsApp ?
That sure sounds like the program is designed to work the way you mention if you test the same version of the app against three different devices.
If you have current iOS on all devices, the next step would be to contact the developer. When they program the upload, they can choose to let the upload happen in the background or in the foreground. The blocking of the UI indicates they chose to use foreground or that your devices all have disabled background processing for that app.
You might check your settings and then if you can't see any way to change the foreground / background context them.