This issue is related to another known problem at our side. It is logged in our Feedback Portal
where you can track its status.
As a workaround I suggest modifying the ExpandBehavior to collapse items only if these items have children:
var sourceItem = item.SourceItem
// Collapse Items only if the SourceItem(GanttTask) has children.
&& sourceItem.Children.Count > 0)
Hopefully this helps and is suitable for your scenario.
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