[NAME]  Figured I'd respond in this thread rather than in the other one where you linked to this one.  What you've found is very strange, and I suspect is a firmware bug that's causing the battery charge policy to apply only to barrel-style power sources, when obviously it would apply to USB-C power sources as well.  Just out of curiosity, have you tried going into the BIOS Setup to confirm that the battery charge settings match Dell Power Manager?  And while you're in there, I haven't seen the BIOS interface of the Vostro 5590 specifically, but hopefully you can find some sort of Battery Information section.  If so, it should show you whether the battery is charging, discharging, or idle.  Assuming your battery level at the time of this test is between the minimum and maximum charge level settings you defined, can you reproduce this behavior in the BIOS where the connecting the barrel-style charger keeps the battery idle while connecting a USB-C source causes the battery to start charging?
My Setup:  Without the dock, the Dell Power Manager is functioning correctly whereby, setting the charge-from 50% & charge-to 90% is observed via the battery status indicators on the taskbar or in the Dell Power Manager App. Normal power supply connector in-use for charging. With the dock, the Dell Power Manager settings appears to have no effect. The result is that the battery status indicators remains "Fully Charged at 100%."   Please advise.
Dell Power Manager - Not working through WD19 Dock Station