When using an LFO in mono mode, all voices should share one single LFO (according to the reference manual). This doesn't seem to be the case.
You can reproduce this behavior with the attached patch file and doing the following. Play a Cmaj7 chord and, while holding the C key, playing an arpeggio in the upper three keys. After a couple of key presses, the chord starts to sound very messy. It sounds as if every key press has its own LFO and they are not in sync with each other anymore. They are in sync again, when releasing all keys and starting over or after a couple more keypresses.