At least at T1, there is a second ATM machine that accepts foreign cards, it is the second machine away from the Citibank machine. I am a Citibank customer, but on one trip, as a simple experiment, I withdrew $200 from the Citibank machine and then the same amount from this other machine. When the two charges hit my account, the amount in USD differed by only 10 cents.

So I believe these machines are pretty much regulated to use the same exchange rate and fees (unless things have changed), and if the Citibank machine happens to be out of order, just use the other one.