P: Smart previews disappear for modified pictures

I generated smart preview for a entire collection and I can see the "smart preview" and "original picture" below the histogram for all pictures. However it happened several times that when I remove the external disk on which raw files are stored, the smart preview disappear for the picture currently in development. I can check now that I have 16 pictures with smart previews missing, these are exactly the picture I modified in the development module. Is this a known bug ?