Imported mesh previews incorrect in Project panel

Hi,
i have this strange problem where Asset previews of imported meshes have a random background rendered into them, often a sphere, a cube or some other meshes from their hierarchy.
The mesh is a direct Blender (.blend, 2.78c) import.
Additionally, the little interactive preview window (mesh selected, shown at the bottom of the importer inspector) has some unusual buffering kind of problem; when i rotate the object it seems to be rendered ontop of the previous preview render and only clears that on mouse release.
I noticed this after the last update (not sure if it was this way before that one though).
So, is my installation broken or does anyone else experience this?

 

Anyone?
Even a default cube exported from blender has a sphere rendered into its preview

 

5.6.2 fixes the buffering background (with a decrease in the framerate of the preview window too) , but does not fix the preview being wrong for subobjects:

 

Not sure what did it, but after about the third or fourth reimport it seems to have magically fixed itself