I have not yet figured out what will start it off. You can tell it is happening by the transport position counter being active.
Once it is playing, the transport buttons will not have any effect. Nor will being in DAW sync.
It is basically just a minor annoyance. If you close the window and reopen, the control is regained.
Additionally, the level meters on the instance sometimes keep moving even if no sound is being played. The midi panic button will stop that.
I have only tested this out with the VST hosted in Logic.