iPhone 5SE doesnt turn off?

Hello,

I am having in repair one iphone 5SE. The device cant be turned off. First i thought it was the power button flex damaged. ( with the old flex you could turn the device on but cant turn it off- you cant turn the lcd black again/lock the screen). So i ordered 2 power button flexes and both do the same, you can turn the device on(when the screen is black or the phone is turned off) but when the phone is on, you cant turn it off, or lock the screen.

I also tried the assistive touch and when you choose the the lock screen option on assistive touch the screen still reamins on?

I dont know how to solve this issue if the assistive touch doesnt work, it should be a software issue??