Mainstage 3 arpeggiator plays 2 notes instead of one

Despite reinstalling MainStage 3.6.6 twice, the Arpeggiator plugin will not play a basic single note pattern. No matter what I settings in Pattern mode it plays 2 16th notes for each key press. I have tried recall default in every conceivable menu and it appears that the default is corrupted or this is a glitch that I cannot remediate. Any suggestions?

Thanks!