From the user guide "If an activity is multi-instance, this is indicated by three short lines at the bottom of that activity. Three vertical lines indicates that the instances will be executed in parallel, while three horizontal lines indicate sequential execution. "
But parallel multi-instance tasks are shown as horizontal lines in Activiti instead of vertical lines.
It seems to be a bug in drawActivityMarkers function in ProcessDiagramCanvas class. First parameter seems to be flipped to false/true. It should be true/false as shown below.
if (multiInstanceSequential) { drawMultiInstanceMarker(true, x, y, width, height); } else if (multiInstanceParallel) { drawMultiInstanceMarker(false, x, y, width, height); }