It all depends upon where the tensioner is.
Most put them between the engine and the first pulley so one pulley has a bigger contact arc then the other.
This oft leads to one pulley not slipping at all and the other one slipping a little undr load.
This condition gets worse as the belt wears.
Check the routing very carefully, more than once I have gotten under a ZT mower to fing some one had reassembled the tensioner in the wrong place.
On a 61" Dane I ended up popping a second tensioner under the mower to cure the same problem.
However this is in general as I am not familiar with your machine and unless the mechanic is familiar, get a manual and check the belt routing your self.