what should I do if one key on the keyboard sticks out slightly? (You can see the backlight under it, which isn’t visible on the other keys).

Welcome!

Depending on Macbook Pro model, that is indicative of some form of impact damage. I learned that the hard way 😱. If you cannot reseat it with modest pressure, the underlying mechanism for that key is likely damaged and needs replacing. Apple will replace the entire upper deck assembly if the KB is damaged. Likely pricey.

You can search for third-party sources that sell individual keyboard repair parts depending on your Macbook Pro sub-model (not all models may have user-serviceable keycaps) but we cannot link to such sites here. Web search for "Macbook Pro keycap replacement" and see what turns up.

The keycap is fully functional (damage is not possible),

I had no doubt it is functional.

I was referring to the little spring mechanism under the key cap. Directly under the black key cap is a "key hinge" made of white nylon or some other flexible polymer. It uses a "scissor" action to return the key to its "at-rest" position after a keystroke. See the image here of the key hinge used on your Macbook Pro model:

Macbook Pro M-series key hinge

(YouTube videos show it better)

If the key cap assembly is struck from the side, the hinge can be stretched, as happened to mine. It may not fully return to its original at-rest level. The key will still work to type its assigned character, but may stand above those around it.

If your keyboard has not suffered a glancing blow as did mine, and the Macbook Pro is under warranty, have Apple evaluate it. A high key can eventually mar the screen.