I think "filled with sugar" would be "umplute cu zahar".
I would translate this as "The apples are not full of sugar." "Full of" is not the same as "filled with".
Why is "pline cu zahăr" not correct, given the English says "filled with sugar"?
It is correct, report it. In fact, they mean different things and the best Romanian translation for the given English sentence is "Merele nu sunt umplute cu zahar".
merele nu sunt pline cu zahăr is correct in fact both are!
you can't, like, open apples and pour sugar in them. "full of sugar" would make more sense. that would be an idiomatic way of saying "apples have a lot of sugar in them".