Thanks mate for the reply.
I thinking of living with the issue as alternative option available to make phone to airplane mode to replicate the switch off scenario., but only thing is if this is a new phone i buyed last week. and i am wondering if it is a hardware issue i just replace the phone.
There are many L720 users experiencing the same issue. I believe this is a really interesting bug and I really wonder what may cause this. Some 720 users say that they fix the problem by turning off the auto brightness setting