Had the same experience briefly returning to an office last year, some very unappetizing Coke Zero. I don't know how much Ace-K differs in this regard or if there's not really enough to matter but... not good.
Was not really something I previously thought of as really expiring in any meaningful sense.
Acesulfame K is not sensitive to degradation at coke pH. In addition to acesulfame K coke zero reportedly contains aspartame as well so that was probably the cause of the off-taste.
Could it be that you just didn't drink cola for a long time? I haven't really drank any soft drink in few years and whenever I try it just tastes like very sugary water to me. And I used to drink a lot of cola in the past.
Was not really something I previously thought of as really expiring in any meaningful sense.