I too have had this. However, I believe I have found a fix.
At the very bottom of the mix window of any/all mixes using keystone, if you turn the "Offscreen" option to "on", it should fix this. It has fixed it when I've had it happen.
Support PDF, settings, and photo of the bug attached. I have only been able to reliably reproduce this on one Mac Pro - my Macbook Pro is fine.