Test steps:

Create a project and add View3D

Import the attached 3D asset to the project

Add component to item library by clicking the "Quick3DAssets.Box" flow tag in QML Types tab

Open the 3D Edit View and move it somewhere other than on top of the Form Editor, e.g to another screen entirely

drag and drop the component to it

-> it is not added either to the navigator or the 3D Edit View

Seems that if the 3D Edit View happens to be opened on top of the Form Editor, and the imported component is dragged so that the mouse moves over the View3D (placed on the Form Editor) while dragging, the component is automatically dropped to the View3D without even releasing the mouse.