Losing charge while plugged in

I recently got the Razer blade stealth 13 with the 11th gen I7 for Christmas. And have noticed it lose battery slowly while plugged into the 100w power supply it came with. I started playing Halo infinite at 100% charge, while plugged in, but after about an hour of being playing in it dropped to 90%, and then the 80's after an additional hour. I know this is a problem with using weaker 65 watt supplies on these models, so should this not happen on my 100w. I'm not so familiar with windows laptops and don't know if this is par for the course. Or if it's an internal fault or one in the power supply. Every game I've tried experiences these drops, at varying degrees, but drops in percent none the less.