Unable to auto-lock in iphone 5

I notice that iPhone 5 (iOS 6.0.2)  is unable to auto-lock with whatsapp or wechat running in the background (i.e.  it appears in multitasking bar) I've checked with whatsapp support. They suspect its OS level issue.

Reproduce steps:

1. Set auto-lock timeout to 1 min

2. Clear all apps in multitasking bar

3. Click whatsapp to enter the app

4. Click Home button back to home screen

5. Wait to see if auto-lock works or not

Am I alone for this? Thanks.