Faux is "mispronounced" in English. English uses a diphthong, whereas French uses a monophthong. Every speaker of every language adapts borrowed words to their phonological system.

It is, in English and in other languages. Sometimes a mispronunciation gets bedded in because people are lazy or the sound doesn't exist in the borrowing language, sometimes because it's funny, sometimes because the pronunciation rules are different and one use catches on to such a degree that it's not worth arguing about (eg the English pronunciation of Paris).

I think faux is actually the outlier there, it is basically a pretentious/euphemistic way of saying phony. It keeps the foreign pronunciation because it is used as an attempt to put on airs (sometimes sarcastically).