You did not specify if the Latitude 5420 is being used on battery only or with its provided power adapter?  The P3421W USB-C port only delivers up to 65W power back to the Laptop, page 31. If using the Latitude 5420 on battery only, connect the provided power adapter to the Latitude 5420 and retest. Open the P3421W Menu- Others. Check the Firmware version. If your currently installed Firmware is older than M2B207, then install Firmware M2B207.
I am using Latitude 5420 with a USB-C connection to my P3421W monitor. Windows 10 very frequently has USB Hub failure. My USB mouse keyboard and my USB SoundBlaster X4 always not working after Windows indicated USB Hub failure. I have to turn off the monitor for 3 seconds and turn it back on again. It resolves for a while before having the issue again. Is anyone having a similar issue?
P3421W, Latitude 5420, Windows having USB Hub failure