Logic X deletes my loops

I have a huge library of drum Loops. From time to time it happens, that some of them are being displayed Grey in the Loop browser. It means that they should be there, but in fact they aren't. When I check the Aplle Loops Folder in the user library, it turns out that These Grey file are no longer in the library. When I restore them from a back up, they instantly work again.

Recently I found some drum Loops in my trash Folder. I guess (it is only a guess) that those were the deleted / missing files.

Does anyone have an idea why it happens and how to avoid it?

Thanks!