I can confirm it's the update to BIOS 1.6.0. causing this problem. After I reverted to BIOS 1.5.3. the button worked again. You can download the installer for 1.5.3. from the driver section of the Dell website. When you run the .exe file the BIOS will be reverted and your power off button works again.
My Dell G3 15 3500's power button is not working as intended. It only works when pressed 15 seconds or pressed when laptop is off. The problem is that it used to shut off my laptop after 4 seconds of holding the button and lock it after 1 press but this functions are not working. This all happened suddenly today and it was working fine yesterday. I have all my drivers up to date and I've checked power options and restored default settings for the plan I was using. I also full scanned my laptop using BIOS and tried restarting but nothing seems to work. Dell G3 15 3500/ i5 10300-H/8gb ram/1650ti 4gb
dell g3 15 3500 power button not working