Hi Jack, I have heard about this as well, you are not taught incorrectly. Let me try to explain it to you: if you pronounce ren with an r like the american r, then it is not entirely correct Standard Chinese.
Instead, you have to stretch your mouth to both sides left right while producing the american r sound and shift the toungh slightly forward. Then, the r is changing a lot and sometimes it really sounds like what we in western countries would feel like zh.
However, comparison of a sound in a foreign language to a sound in a language you are familiar with is never 100% match. Try to listen and observe carefully, eg. watching the Chinese news.
Good luck, Valeria