OP- I have the same mower, a 2010 model & never had this same problem you are having until about a month ago -and this is after 3 yrs of light commercial use (that it's not designed for).
I just posted for some advice/help in the repair forum. I'm new here, BTW.
My dealer has been great to me & has worked hard to correct the cut problem, but we've run out of reason why it suddenly started cutting poorly.
It is not inherent in this mower I know that, as my next door neighbor just bought a brand new one.
Also, the ZT2548XL was discontinued a couple of yrs ago & replaced by the ZT HD, if I recall correctly.
For what that's worth....
I'd be grateful for any advice/solution anyone might have for this problem.....