Where did the spelling/pronunciation "Sephardi[m]" come from, vs. "Sefaradi[m]"?

See title.

I’m of the opinion that while spelling of transliterations can kinda be whatever, the pronunciation of words does actually matter; so, not pronouncing the extra patach in “sefaradim” just sounds objectively incorrect.

I’m curious where the difference emerged, and why it has lasted as long as it has.