Complex sentence: Upon losing the feature of length, Latin/ī ū/merged with nothing, but the "short" high vowels, front and back, merged with the "long mid" vowels: thus, Latin/i ē/are uniformly reflected as PWRom.
Split into simpler sentences: Upon losing the feature of length, Latin/ī ū/fell together with nothing. But the "short" high vowels, front and back, fell together with the "long mid" vowels: thus Latin/i ē/are uniformly reflected as PWRom.