may i ask, where did you depart from?
if you depart from international, you cannot use AMEX..
but when you depart from within malaysia, no problem using AMEX

i can still see the option when using iphone apps..but i did not click confirm..
you can't see the option at all or payment fail after you key in the cc details?

Uniqlo branches uses CIMB terminal, accepts Amex. Jaya Grocer near my place, sometimes uses the CIMB terminal as well to accept AMEX. Tldr, if the banks supports Amex, the terminal will support amex. Nothing to do with the terminal’s hardware.

Hong Leong and CIMB also work but they need an additional step on their part and possibly payment fee to Maybank to work. Even then it's only for Jaya Grocer's. I've never seen a UOB terminal work for AMEX. I've also seen Maybank fail recently which is freaking weird because they would have had to request for AMEX to be disabled since it works out of the box when they sign up for it. Mine didn't work for a local Subway which was using Maybank but then I went straight to Family Mart afterwards to see if there was something wrong...and theirs worked without an issue.
Only Maybank's work by default. Every other bank's terminals needs to allow it before it works and even then the Merchant needs to request the feature from their bank. I've seen some disable AMEX acceptance on their Maybank terminal due to the higher fees.