The event tooltip and editor do not always display the same end date. This can be observed in this demo https://bryntum.com/products/scheduler/examples/frameworks/vue/javascript/tasks/dist/
When a new event is being dragged onto the scheduler e.g. from May 7 to May 11 inclusive the tooltip displays May 12th as the end date. The event editor also displays May 12th. When the event is created though the tooltip changes to May 11th as the end date and the event editor still displays May 12th