The plug icon indicates charging is on hold. It's expected behavior if it's normally and regularly plugged in for extended periods of time at the same general times of day. The Mac learns from your charging habits and optimizes the charging to preserve battery health.
It will manage the charge and not charge to 100% at those times it spends plugged in until it calculates it has to charge to full because you normally unplug it.
If you don't want it managing the charge levels like that, you can click on the battery icon, then on Battery Settings then on the (i) icon next Battery Health, and then turn off the switch for "Optimize Battery Charging"
click here for details ➜ If battery charging is paused or on hold on your Mac - Apple Support