I was on the same problem that the power button not responding to the short press to enter sleep mode/ wake. It will work only for "the slide to power off" option. But I noticed that after specific time it is going to sleep mode. So I realised it is definitely a setting.
I have done all the things saying to do in the hardware side like covering with tapes, new flex cable, etc, etc
None worked for me
THIS IS JUST THE SETTING OF YOUR IPAD. ( mine is the ipad mini4)
'''GO TO SETTING
-> GENERAL scroll down to
flip that bullet to make sure it is off. Now try..
The power button will work as normal. it is nothing wrong with the new screen or digitizer you replaced.
if the home screen is not showing up at all even through home button and power button then it is definitely the small sleep wake sensor. you may forget to transfer it to your new digitizer. check it
Now the software update is making me crazy that unable to verify update.
if you have any suggestions please share it