When you add sugar to it, your cup of coffee becomes not only sweet, also flattening.

Question

Which word is not grammatically used in this sentence ?

Answer 1

What does the sweetened coffee flatten? The word is not the correct word.

Answer 2

The words after the comma should be "but fattening."

Answer 3

......your cup of coffee not only becomes sweet, but also flattening.

or
.......your cup of coffee becomes not only sweet, but also flattening.