16 Aurora AC16250, problem with the power supply diode

No it isn't, but more detail requires knowing the system model.

Dell Alienware 16 Aurora AC16250

https://drive.google.com/file/d/1oygLWk6bhRL6_BOjoRXprfbGHdZZt46-/view?usp=drivesdk

According to Dell, this may be normal. There's some reverse voltage (minimal). But it's strange because when I unplug the cable from the laptop and plug it back in, it doesn't light up. Or if the laptop is off and then charging under power and I unplug it, the LED stays on until I turn the laptop on, then it turns off.

I would press for a warranty replacement of the adapter -- if that doesn't solve the problem, it's likely a faulty system board.

I can understand some stray voltage but not enough to illuminate an LED on the adapter.

Does the system perform normally on battery power?

Well, the system is working normally, both on electricity and on the battery. There's nothing wrong with it except that pesky diode. Service says it's nothing serious.

The problem is still not resolved. I've been trying to resolve the issue with Dell for almost two weeks, but no action has been taken. I've called the hotline, sent messages on WhatsApp, and nothing. Everyone says something different. A new laptop, but a broken one...

If the system is less than 30 days from shipment, request a refund or  replacement system.