The plural of quantum in Latin is quanta.
Since it's an English word, no matter which language it originated from, its plural form follows the rules of the English grammar.
Else-wise, the plural of the word 'metaphor' would be, as in Greek, 'metaphorai'. But that's not the case, because that way we would have to find every time the origin language of every word and form the plural based on that language's grammar rules. But that's not the case in any language. Much less in English.
Ex. the plural of goose isn't gooses, it's geese. The plural of mouse isn't mouses, it's mice. (on the other hand, the plural of house is houses, not hice).
English plurals are a weird bunch, and saying they follow the "rules of English grammar" is very much incorrect