The Not Charging message that you see is most likely displayed by the Battery health management feature build into macOS Catalina 10.15.5 and later, and is available on MacBooks with Thunderbolt 3 port.
The Battery health management features is intended to slow down the chemical ageing of MacBook battery by analyzing the battery temperature history and charging patterns, and intermittently stopping the charging.
You can toggle the Battery health management feature by going to System Preferenes app → Energy, and clicking on the Battery Health... button and unchecking Battery health management.
Apple recommends you leave it turned on, but lets you turn off the setting.
You can learn more about Battery health management by reading the following Apple Support document: