Dell XPS 13 2 in 1 7390 charging using via a USB C multiport adapter

@sic_99  As the saying goes, it's not a bug, it's a feature.  Multi-port adapters that support USB PD passthrough skim some power from the source for themselves before passing through the remainder to the system.  The adapter needs power to run its own internal electronics and provide power for anything you might plug into the powered ports of the adapter, such as USB ports.  If you don't have a power source connected to it, the multi-port adapter pulls power from the system.  But when you DO have a power source connected, the adapter can't provide power to the laptop and draw power from it at the same time -- so instead it simply skims some power off upfront.  You could avoid this by connecting the power adapter and multi-port adapter separately, and that will avoid the warning, but it won't really change the end result because in that case the system will be receiving full power but will still have to send power out to the adapter.  The multi-port adapter has to get power from somewhere.  And sometimes that will be arguably WORSE because some multi-port adapters perform better when a power source is attached.  For example, they might offer more power out of their own USB ports, so when running with a power source connected they might be able to run USB-powered external hard drives, whereas when running from system power they might not be able to.  This depends on the adapter.  But even if you had an adapter like that, then is avoiding the power warning worth making the adapter less useful?

The other way to avoid this is to get a higher wattage USB-C power source.  The XPS 13 is designed for 45W.  Some multi-port adapters skim as much as 15W.  So if you were to get a 60W USB-C source, there would still be 45W available even after the skim -- so your system gets full power AND your adapter runs at maximum functionality, because you're introducing more power into the overall environment.