The Latitude 5421 shipped with 90W and 130W external AC power adapters. The P2422HE USB-C upstream port only delivers up to 65W back to a Laptop. Perhaps this is why the Latitude 5421 shows the yellow triangle?
I have a new Dell P2422 connected to a Dell Latitude 5421 using usb-c. Everything works fine except that the battery symbol in the sys tray shows a yellow triangle. The battery has charged with this setup and shows to be at 100%. I checked Dell Command Update to see if there were any updates to fix the issue but there were none. The yellow triangle is annoying though.
P2422HE, Latitude 5421, battery yellow triangle