Same here. When I started working from home, I set up an external monitor with an HDMI-to-USB-C adapter. Whenever it's plugged in, my battery goes down about 4% per hour with the AC adapter plugged in.
I've only ever seen my laptop's battery discharge while plugged in via one of those USB-C hubs which allow PD "pass-through". But have yet to find one that passes all the power through.
The adapter claims some of the power for itself and downstream USB devices. I think it has to reserve power statically, so it's always claiming 5W or so.
