iPad Air power/lock button doesn't work - very weird!

I have an iPad Air Wifi 1st gen for screen repair. According to customer, the power/lock button worked fine before. I smoothly removed the old screen, put on a new screen. Everything works perfectly. However, I notice the power/lock button doesn't work properly. The symptom is very strange, never seen something like this before:

When I pressed power button, the iPad turned on perfectly fine. When I pressed power button again, it won't go to sleep mode. It doesn't matter how long I keep it pressed, nothing happened. The "slide to power off" won't come up. After 2 minutes, the iPad went to sleep mode automatically (because the auto-lock was set to 2 minutes). If I press the power button again, the iPad will wake up. But it won't go to sleep again until 2 minutes passed. If I press and hold the power button and home button at the same time for a few seconds, the iPad will soft reset and restart as normal.

I thought the power flex might have problem, then replaced it with a known working power flex. But it made no difference! The power button can only wake up the iPad, but it can't put it to sleep or turn it off. I reset the iPad (haven't restored it yet, just reset) to original factory setting, again made no difference!

Any help will be highly appreciated. It almost drives me mad!