Oh wow - now that I've tested a number of USB-C cords, I was wrong. It was a 65w USB-C charger from an older Lenovo Yoga that had the issue. The USB-C dell charger for my Latitude 5420 is fine, as is my 65w RavPower brick and 45w Anker brick. There must be some sort of weird shenanigans with that Lenovo charger!   Thanks for the replies, [NAME], I probably wouldn't have second guessed the charger itself since it works fine for my other laptop and chromebook. 
I just purchased a refurbished Latitude 7280. As I'm testing it out, I have found that the built in keyboard stops functioning correctly if I have a USB-C power cable plugged in. It doesn't have the issue if the proprietary plug is connected. It powers fine with the USB-C cable.   Has anyone experienced this? Any ideas on how to fix it? I bought this to be a light secondary travel laptop, but having to carry around a separate power brick would defeat the purpose. Thanks, Kevin
Latitude 7280: Keyboard stops working correctly when charging over USB-C