The unstressed word ending <ex> or <exe> is pronounced eks and is not weakened to ɪks or əks. Here are some example words:
annexe circumflex complex duplex multiplex index reflex simplex vortex
The same is true of quite a few trade-names, like:
Andrex Gortex Kleenex Rolex
There is a separate tip on the place-name ending <sex>.