this is a unknown problem.
If level is not hot enough, pitch shifting may behave incorrectly. This is also tied to the proper type of connection according to the signal level at input stage (XLR for +4dB line level/ 1/4" jacks for -10dB instrument level).
How many leds are lighting up on the front panel?
You don't need any change between mono and stereo setups to make pitvh shifting work correctly.