Except that it’s spelled “Latex” with all letters from the English alphabet and there is already an existing word with that spelling, therefore it is pronounced the same way as that word. You don’t pronounce “Laser” as “Lah Seer” even though the “A” comes from “Amplification” and the “E” from “Emission”. Once it became a word, it was pronounced using standard English pronunciation rules.

Latex, like the rubber stuff.