@Ultraspider Sorry that Spinebot's reply wasn't really helpful, I've just deleted it so it doesn't confuse others.
My best guess it that the problem originates from the newly exported skeleton now requiring fewer atlas texture pages than the previously exported one, while your prefab still has references to the old materials, leading to the first entry being set to null since the material and texture with index _6 no longer exists.
You could have a try whether hitting Reload next to the SkeletonData property in the SkeletonAnimation inspector resolves the issue, re-populating the list.
If this does not resolve the issue, and you still see an incorrectly populated list, perhaps the _Atlas asset has not been regenerated and repopulated properly. Could you then please share a screenshot of the _Atlas asset? Please also share a screenshot of the folder that contains your problematic skeleton's textures and materials.
Another typical guess would be that there are leftover .meta files of the materials or textures around, however I doubt that's the case given your Inspector screenshot.
