Hi Jame,
Thanks for your answer, but I’m not sure it applies to what I describe. I’m not talking about migrating stuff between projects but of importing assets (textures and fbx mostly) stored on my harddrive to a project.
Your answer though gives me an idea for a workaround: I will create a project in 4.10 or 4.11, import my stuff and then migrate it to my 4.12 project. If it works, it will still be a workaround and I still think that this behavior is probably a side effect of another change made to the drag and drop to content browser functionality.