An entire surah need not end every verse on one metre or rhyme. One subject may require extended exposition, another a sharp warning, and a third a narrative or argument unfolding in stages. No single acoustic mould fits all rhetorical situations equally well.
Some passages move in the manner of public exhortation or wise counsel, while others develop through a style resembling a letter or sustained conversation. Forcing every clause to close with the same rhyme could burden natural expression and obscure meaning.
Balance: pleasing sound matters, but not at the expense of clarity. When the verse ending changes, the task is to understand its appropriateness, not presume a defect.