Well it’s a bit odd! As the charger is not charging the battery at the correct power.
So either the handshake between the computer to charger is not correct for some reason or the cable is bad so the handshake is not getting thru properly or the power lines in the cable are damaged.
Let’s try a second USB-C cable ideally a real Apple one or one that is designed to power a laptop. Not all USB-C cables are fully wired or intended for charging. So do be careful on what you use.