I would strongly recommend that you do two things;
First check the charging port for debris and blow it out or carefully remove anything you may notice there. If your device is acting as though something is plugged into it when there is not it could be a sign that something is in the port that causing contact between two terminals and this poses not only a risk to the device but also a personal risk as short circuiting a battery can lead to explosion.
Second I would take the device to your carriers service center, if possible, and have a trained tech take a look at it to verify that the device is safe to use and charge. While it could be a bug or other issue, the fact that you have to have the device turned off to charge is a sign that the issue is not software related. regardless of a software bug being hardwired into the battery should cause it charge no matter what. The circuitry that controls charging is between the battery and the charger and the battery not charging means that one of the two may be damaged. In this case probably the battery since it is reading charging when it is not connected to anything.
More questions;
Have you noticed the device getting hot at any time? While using or charging?
Have you noticed a reduction in battery life?
Have you checked the plug on the cable? While USB-C is a much better connection than Micro USB and far less likely to become damaged in comparison to Micro USB, the opening on both the plug end and the device port are much bigger and more open and either can pick up debris without the user necessarily noticing.
How long have you been noticing this?
You mention that when not plugged in you are presented with only one option under the USB symbol, which is to charge, but do you get other options when actually plugged in to a USB that is plugged into a laptop or PC? Plugging it into a wall outlet shouldn't offer options. Only when connected to certain devices do these options appear, this is why I'm specifically asking about what you see when you are plugged into a laptop or PC.
Let us know what you find.