@LZL Thanks for the additional information. Unfortunately questions 1 and 2 were not quite answered for me:
What does your initial skeleton setup look like material-wise? What materials are active before repacking?
Here I would need to see the Inspector settings with your SkeletonAnimation GameObject selected, and the materials showing at the bottom of the Inspector. You have shown the Material selected in the Project window, which helps a bit but does not yet answer everything that I need to know.