In "Ciasteczka są smaczne", "smaczne" isn't actually neuter - after all, "ciasteczka" are plural.
We have two plurals, 'masculine personal plural' and 'not masculine-personal plural'.
masculine noun describing a person -> its plural is 'masculine personal'.
masculine noun not describing a person -> its plural is 'not masculine-personal'.
feminine or neuter noun, regardless of what they describe -> their plurals are 'not masculine-personal'.
In the Nominative case, the 'not masculine-personal' forms of adjectives are identical to the neuter singular ones. But if for example the noun here was "ryby" (plural of feminine "ryba", fish), then it's also "smaczne", although a singular "ryba" is smaczna".