HP OmniBook Ultra unwanted shutdown.

I've had an HP OmniBook Ultra for a few months and it has been just great. I partitioned the SSD and installed Linux Mint and Ubuntu alongside Windows, with Mint as my preferred OS. Instead of the OEM charger I've been using an Anker USB-C charger with USB-C to USB-C cables. Even though the charger doesn't say it provides the 15V / 4.3A combination, the laptop has charged and worked just fine.

At my home office I plug a USB-C cable from my 49" monitor to the laptop, which charges it just fine.

The other day I lost my charger and bought another, though it isn't the exact same one. Everything seemed fine until the laptop initiated a shutdown sequence. It then boots up, but after a few minutes (sometimes as much as an hour) it will do a shutdown. This happens in all three operating systems.

A friend suggested there might be something wrong with a thermostat on the motherboard, and the laptop thinks it's overheating? The laptop is always on a hard surface, table or desk, with proper ventilation.

Also sometimes when the laptop starts, the screen stays dark and the internal fans whiz at high speed.

I'm currently traveling and had to buy a cheap Chromebook to get through the next six days on the road, but I sure would like the OmniBook to return to working properly. Thanks in advance for any ideas as to what is going on.