Well, these transmissions are completely controlled by solenoids.
It could be one of the solenoids, the wiring that connects that solenoid to the computer that controls it, or an internal mechanical problem.
Do you know which solenoid(s) were replaced, what the trouble code(s) were?
Looking at the solenoid engagement chart, the only one that changes state during the 1-2 upshift is "shift solenoid D" which goes to full-open.
(if the cart is left in "D", manually upshifting involved solenoids A, B as well) .