According to Apple, “Depending on its settings, your Mac might temporarily pause charging to help calibrate battery health management, a feature designed to improve the lifespan of your battery. When battery health management is turned on, you might occasionally see “Not Charging” in the battery status menu of your Mac, and your battery’s maximum charge level might be lowered temporarily.”
Apple adds, “This is normal, and it’s how battery health management optimizes charging. Your Mac resumes charging to 100 percent depending on your usage.” These battery optimization features are something that Apple has introduced to its products over the past few years, including the iPhone.
Given that batteries in our devices are harder to replace compared to last time, Apple has taken to using software to better manage our devices’ battery health. You can opt to turn off the feature if you’d rather not encounter this in the future, although it could be detrimental to your laptop’s battery health.