Why is the plural form of 'cupful' not 'cupsful'? [closed]

The plural form of cupful is cupfuls and cupsful? Shouldn't we be pluralizing the noun (cup) instead of the adjective?

Cupful is a noun and follows the normal pluralisation rule, so the plural is cupfuls.

That said, cupsful does appear as an acceptable alternative in US English dictionaries https://www.merriam-webster.com/dictionary/cupsful

And the UK English Cambridge dictionary lists it as a US English alternative https://dictionary.cambridge.org/dictionary/english/cupsful

Obviously cupful is a compound of cup and full, so when separating the words you still pluralise the noun, e.g.: 'two cups full'

Cupful is a unit of measure. Two cupfuls doesn't imply two physical cups. You could measure two cupfuls of flour with one measuring cup, pouring the contents into a bowl. Two cups full of coffee does indicate two physical cups.