Dell Inspirion 16 Plus 7620 not charging

A non-charging issue can be with a power adapter, a faulty DC power-in port, a failing battery, or worse, charging circuit on motherboard.

For starter, tapping F12 at boot and select to run a diagnostic scan to detect any hardware issue.  If there is nothing specific, you can monitor when the non-charging issue happens, to to Task Manager, then select Performance tab to see if the processor gets downclocked.  That is a symptom of unrecognized charger, resulting in BIOS limiting the CPU performance and blocks the battery charging function. 

The first and basic resolution is checking if the power adapter was firmly connected to power port.  Next, testing with another power adapter, replace DC power-in port (not applicable with USB-C).

Second and basic resolution is performing battery calibration.  This will allow the Windows operating system to recognize the baseline for battery charging.  Review this article https://www.dell.com/support/kbdoc/en-us/000205559/how-to-perform-a-battery-calibration-cycle-on-a-dell-laptop

@Chino de Oro​ Thank you for the advice.

I ran the onboard diagnostic - when charger was connected, but not charging, I got "Charger - unable to detect charger". Later, when I turned the laptop off, plugged in charger, and ran the diagnostic - "All tests passed successfully".

My battery health is excellent, charging mode is set to "Always AC", but I tried "Adaptive" also and it didnt fix the issue.

I use the original USB-C charger that came with the laptop. The laptop also has jack connector, might try to buy barrel charger and see if the issue persists.

I do not have a problem where I get "plugged-in, not charging", it is either detecting charger and charging, or not detecting charger at all. It only starts charging when the laptop is off, but then continues charging when laptop is on. From battery usage history, it seems the laptop stops charging at 100%. So the issue might be some kind of software  battery saving/life prolonging thing, but apart from "charging mode" I cannot find other settings.

On older systems, users could customized battery charging mode with Dell Power Manager.  Your system, however, is using My Dell Application.  You may use it to optimize the battery performance and to prolong the battery health.

Using a barrel charger may resolve the issue  https://www.dell.com/en-us/shop/dell-45-mm-130-w-ac-adapter-with-2-meter-power-cord-united-states/apd/492-bdbl/pc-accessories