We recently had a customer contact us about a strange issue where his rack would intermittently start emitting out-of-tune audio. The customer was concerned that something in Forte or the audio driver was the cause of this.
After more research, he discovered that he had a MIDI keyboard controller that was generating pitch bend MIDI events occasionally that he wasn't controlling. It turned out that the controller had a glitchy pitch bend controller and would signal bend messages even when it wasn't being pressed.
Forte doesn't manipulate the audio signals it generates except for volume and mix. Nothing in Forte alters pitch, so any pitch issues in a rack are most likely something like the above described pitch bend situation or another MIDI modulation controller which might be affecting a plugin's tuning. It is also possible for it to be an audio driver issue with clocking but unless your audio device is driven by an external clock this is unlikely.
You can use the MIDI Monitor to help diagnose issues like MIDI controller and pitch messages.