We always need reasons to have event started in some specific date. If event has no dependencies or constraints, it will start with project's start date (In case of using default forward schedule direction). If there are dependencies, event start date will be calculated according to those constraints.
When you manually move an event on a TimeAxis, an algorithm applies constraints (reasons) to have that event in some specific date, independent of (or together with) dependencies. So, it applies constraint date.
Full description of events scheduling https://bryntum.com/docs/scheduler-pro/guide/engine/schedulerpro_events_scheduling
You could try to use another constraints, as example make your event manually scheduled https://bryntum.com/docs/scheduler-pro/api/SchedulerPro/model/EventModel#field-manuallyScheduled
Btw, I don't see any constraints applied after I resize an event. How exactly did you get constraint date applied after resizing?
All the best,