Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.
There is no need to be confused over this. The plural of miskeen is masaakeen. The use of the plural masaakeen in this context does not make any difference in meaning since it refers to the phrase wa ala alladheena yuteeqoonahu (those who cannot afford to).
Such grammatical constructions are common in the Arabic language. For instance, one may say: rakiba al-nnaasu dawaabbahum (people rode their camels or mounts) although each person is riding only a single mount.
There is a consensus among scholars that each person ought to feed one person in place of a day’s fasting he or she has skipped. So, the variant readings do not make any difference.
Almighty Allah knows best.
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