Yeah, in Welsh the stress is usually on the penultimate syllable, and so for the majority of words, i.e. with penultimate stress, the rule you mentioned is correct: penultimate and preceding y is usually [ə].
Prynhawn however is stressed on the final syllable. (The h is the clue to this.) So the rule doesn't hold.
By the way, a common informal abbreviation of the word is p'nawn [pnaun].