Like many others I got the undervoltage warning on a pi3b+. I see most answers blame a poor power supply. I plugged mine to a bench power supply to test this, I set it to 4 amp and kept getting undervoltage warnings until I supplied 5.5V. Is it possible that the rasbpberry pi is actually defective and needs more than 5V (my usb power supply is actually providing 5.10) ? Is there anything I can do to investigate this further?
There is nothing wrong with the Pi3B+ power supply - it is an excellent design (which you can find documented on the Foundation site). Some few boards have had failures of the 3.3V rail.
The many power problems Pi users have are a combination of poor power supplies and inadequate cables. If you are getting 0.2 drop on half of your cable it is rubbish.
Increasing the voltage to compensate for inadequate cables is poor practice and potentially dangerous.