Hello Steve,
Thank you for your answer.
The virtual items appeared when we changed a sequence number of the Extended by relation. However, the virtual items disappear when I open the model again and confirm it after I save the model.
For now, we will ask our customer to try the method that you suggested. In order to fundamentally solve this problem, we are going to need to review the relationship of objects.
I greatly appreciate your advice.
Thanks,
Hifumi