USB-C monitor draining battery with Apple charger connected

It's certainly possible, especially if there's an issue with the two USB-C devices (laptop and monitor) negotiating power delivery. That's also something that could be affected by a system upgrade, like I suspect with the macOS 12.1 update.

I have tried connecting the stock Apple charger when this happens and it doesn't have any effect. Maybe the laptop is simply relying on the display's power by that point and ignoring the "extra" charger? Unfortunately, there doesn't seem to be any way to disable power delivery on the monitor, other than switching to HDMI or DP and losing the nice one-wire setup with all of my USB accessories running through the monitor.

I don't normally leave both connected to the MBP. 🙃 I only connected the Apple power supply to see if it would help when I noticed that the display connection wasn't charging the battery, which is used to do reliably.

Through the years, I have always left my laptops plugged in while in their "normal spot" near the charger. I'm not worried about its affect on the battery's longevity. I just want the computer to charge when it's connected to a 94W power source, not drain more rapidly than if it wasn't plugged in at all. I've seen the battery drain in less than hour with very light use, while connected to power.

As a bit of a follow-up, I've noticed a few things that might be relevant. First, I've noticed that if I connect my laptop with the lid open instead of using the external monitor by itself with an external keyboard and trackpad, it's more likely to charge, at least for a while. Sometimes that lasts for 15 minutes, occasionally it lasts for days.

And second, I have a second 2019 16" MBP as my work laptop, also running macOS 12.1 but with the 5300M GPU instead of the 5500M in my personal laptop. It charges much more reliably but it still occasionally has short periods (maybe 15-45 minutes) where it discharges before starting to charge again. Its battery rarely gets below 90%. Based on that, I'm temped to say that my personal laptop is defective. But this morning, my work laptop ran itself almost all the way down while connected to the monitor.

The monitor could also be the issue but if that were the case, I would expect both of my 2019 16" MBPs to behave the same when connected to it. The computer is a variable in this.

I've seen other threads about this mention problems with other devices negotiating USB-C power delivery, where a phone plugged into a laptop will run its battery down trying to charge the laptop. Since the laptop's battery runs down so much faster when connected to the monitor, I wonder if something similar is happening, where the laptop is running itself down trying to deliver power to the monitor. The monitor seems to charge other USB-C devices reliably.