After mouse-up on dragging a task, it snaps to the closest date-boundary according to the zoom level. This is as expected. However, if the task has sub-tasks, they are not snapped but left in their current positions.
To test this, go to this sample:
docs.dhtmlx.com/gantt/samples/08 … dates.html
Drag the main task, Project #2, less than a day in either direction. (A handful of pixels are enough to demonstrate the problem.) Notice that Task #1 and Task #2 follows. Release Project #2 and it snaps to the closest day boundary. Task #1 and Task #2 stays put, with no snap happening. (This can be repeated, making both sub-tasks moving further and further away.)
I imagine this could be a simple bug of the demo, but it feels more likely that it’s a bug in the Gantt module.