“Sneeze” or “喷嚏” is a pretty difficult word to write in Chinese in terms of number of strokes and its internal components. I’m not surprised people wouldn’t know it off the top of their heads. It’s like if someone asked you to spell “unnecessary.”
The first, 喷 "erupt", is not exactly common either but is at least used in a few other compounds like 喷水 "fountain".
English orthography is terrible (i.e. a single vowel can be a half-dozen letters), but there's a limit to how complicated it can be to write a word that one knows how to say.
(I'm pretty sure this isn't eye dialect: I consulted the rhyming dictionary a lot, to make sure I was swapping spellings between two words with the same phonemes. I also tried to avoid reanalysis, though some of these words might not quite achieve that.)
I would be quite surprised if someone couldn't spell that word, unless it was a child.