New MacBook Pro draws down power when plugged in

Your computer performs best when connected to AC power. It can use the full output of the Power Adapter AND when doing especially challenging work will also freely "borrow" power from the battery. In some cases, the charged state may even decline during stressful work.

When used only on battery, your computer has no extra cushion of power, and will perform more slowly. However, for ordinary non-stressful tasks this may not be objectionable (possibly not even noticeable.)

In general, you should ALWAYS connect AC power when it is possible to do so, and only run on batteries (which will be somewhat slower) when no AC sources are at hand. There are three micro-controllers cooperating on battery and charging issues, and your Mac will NEVER over-charge.

A charge cycle is ever-so-slightly destructive to batter longevity. When operating as designed (and not using Battery Health Management) battery charge level is allowed to decline to about 92 percent level before initiating a recharge cycle to top up to about 99 percent.

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Catalina software 10.15.5 for MacBook Pro with T2 chip (2016 models and later) includes a new feature called Battery Health Management. Based on your usage patterns, this widens the hysteresis to initiate a charge cycle at a lower level, and stop before 99 percent.

About battery health management in Mac notebooks - Apple Support

https://support.apple.com/en-us/HT211094

This relaxes the set points around re-charging (based on your usage patterns) and can improve long term battery lifetimes. When active, recharging may stop short of 100 percent charged.

It's new behavior designed to maximize your MacBook Pro battery's useful life.

Read About Mac notebook batteries - Apple Support.

If you don't like this pattern of discharge to 90% followed by rapid recharge to 100%, you can disable it in Battery Health Management. But it is supposed to extend the life of your battery over years of use, as opposed to having it be plugged in most of the time and pegged at 100%.

Rebooting the computer interrupts the pattern and will result in it immediately charging back up to 100%, regardless of where it was in this battery charge management cycle. But some hours later, it will restart the slight discharge followed by full charge pattern again. Again, you can disable it in the Energy Save Preference Pane by selecting Battery Health Management and turning it off.

After I understood what was going on, I left it on.

It could be a slight disadvantage if you were to have to pack up and take your laptop somewhere right when it was at 90%. But I figured that's a minor inconvenience and won't happen much anyway.