Multiple subgraphs cannot be dragged to the system

How to reproduce:

1. Open the attached package
2. Open Subgraph.vfx
3. Select System.vfx, Operator.vfxoperator, block.vfxblock in project window
4. Drag and drop them to the Subgraph.vfx graph view

Expected result: All the available selected asset are dropped to the Graph View

Actual result: Only one asset is dragged and dropped