Back Button Acting like Recent Apps button

I recently replaced the phones LCD screen to find out that the back capacitive button is acting like the Recent apps button. It doesn't respond sometimes and if it does, it works as the Recent apps button.

Read few online reviews and most of them pointed me to replace the"Dock Port flex cable Assembly (daughter board)". Did that, but of no use.

Not sure whether this is a hardware a software issue.

Any help would be greatly appreciated.