naqtn
Just after alternating Udon program asset, pasted nodes go into previous asset if pate operation invoked by "Edit > Paste" menu or "Ctrl+V".
Reproduce steps:
  1. Prepare two Udon program assets. Put nodes on 1st one.
  2. Select nodes and do copy operation.
  3. Alternate Udon program asset by clicking "Open New Udon Graph" button on UdonBehaviour inspector of 2nd program asset.
  4. Do paste by "Edit > Paste" menu or "Ctrl+V". (NOTE: Don't click UdonGraph editor before paste)
  5. You'll see nodes do not appear on UdonGraph editor.
  6. Alternate Udon program asset to 1st one. You'll see pasted nodes.
---- Details
I dug because it's strange. I found system native event handler (UnityEditor.EditorApplication:Internal_CallGlobalEventHandler()) seems to call UdonGraph.OnUnserializeAndPaste() on previous instance that holds 1st program asset. (What a magical native implementation!)
This is side effect of "close and open workaround" to alternate asset (on UdonGraphProgramAsset.cs).