That’s what I remembered from high school but I read somewhere… (on the internet so it must be true)… that the syllable rules are limited to the Japanese versions. The brevity and the provision of a seasonal word remains consistent across the format.
The national associations are more flexible and more importantly, the greatest haiku poet of all time, Basho, said that it was more important to write a good poem that to strictly follow the rules…
“Do not follow the ancient masters, seek what they sought.” However strong his opinions and theories, Bashō’s primary allegiance was to the living moment and its accurate, full-hearted presentation. Of the formal requirements of haiku, he said, “If you have three or four, even five or seven extra syllables but the poem still sounds good, don’t worry about it. But if one syllable stops the tongue, look at it hard.”
― Jane Hirshfield, The Heart of Haiku