1) What happened
When importing assets from the asset store while the editor is set to serialize assets to text, the asset does not get serialized to text

A workaround is to set the project to force binary then set it back to force text, but it's annoying to do once your project grows

2) How can we reproduce it using the example you attached
No files attached, but you can check this in a new project by setting the project to force text, then importing a package like the detonator framework, and then opening a prefab in a text editor