With the keyboard frame removed, you can now see the real "story". The keyboard material that you see under and between the keys is very sensitive to foreign objects and liquid.

Check under and between the keys for any type of liquid residue, sticky candy, soda, juice, etc. Any little drop will show.

Sometimes you can look at the keyboard under light so you can move the keyboard around until you get a good glare using the reflection of the light that finds the shiny liquid or white residue patterns against the black background of the keyboard.

You can always take this time to try and air can any crumbs or debris

Any liquid damage or keys not working due to damage like this please replace the entire keyboard. Also, just a note Dell does not sell the keyboard frame separately.