It is a lie that the rhythm of haiku is based on the syllable. Actually, it is based on the mora. A syllable can have one or two morae. I'm unsure whether it can have three? So the word yube (twilight) has three morae but two syllables. So if you have been taught that certain Japanese line have to have 5 or 7 syllables you have been seriously misled.
A syllable with more than one vowel will have a long vowel, a diphthong, or the final consonant n.
The confusion comes because people don't now what a mora is, and because the 1-mora syllable is the most frequent type in Japanese.