The problem lies in the provided project's code. They use bindItem to add the content instead of only binding existing elements. They should be using makeItem for element creation, and bindItem to assign values. BindItem is called multiple times as items are virtualized by the ListView, so adding elements there is not recommended, or should be reverted by a unbindItem or covered by null checks.
For more information, see https://docs.unity3d.com/Manual/UIE-ListView-TreeView.html