Does the dell U3421WE monitor's 90W power delivery (USB-C reverse charging) hurt my MacBook Pro's battery?

Solution 1:

The answer is right there in your screenshot:

Charging On Hold (Rarely Used On Battery)

It does this because holding a full charge of 100% tends to stress the battery. By keeping it a bit lower, it prolongs the battery's useful life.

You can turn off this behaviour in Settings -> Battery -> Battery.

Solution 2:

Is there anything wrong?

No.

The explanation for the behaviour you are observing lies in the following Apple support document:

• https://support.apple.com/kb/HT212049

If battery charging is paused or on hold on your Mac

With macOS Big Sur, your Mac learns from your charging habits to improve the lifespan of your battery.

In macOS Big Sur, Optimized Battery Charging is designed to improve the lifespan of your battery and reduce the time your Mac spends fully charged. When the feature is enabled, your Mac will delay charging past 80% in certain situations. Your Mac learns your charging routine and aims to ensure that your Mac is fully charged when unplugged.

You likely have Optimized battery charging enabled under System Preferences → Battery → Battery

As you have already mentioned that you keep your MacBook plugged-in all the time, with the aforementioned setting enabled, your Mac is waiting to finish charging past 80% to optimize battery aging.

To disbale this behavior and keeping the battery charged to 100%, you can uncheck Optimized battery charging.