Material Thumbnail in library preview blank

[Renata]

Hi,
Not sure if there is already a fix for it, but when creating some materials particularly with higher size/resolution opacity maps the material thumbnails tend to be blank whereas with lower size/resolution opacity maps (or simply if opacity is ticked off) thumbnail renders normally.
In the attached picture you can see , "Created material 1" is rendered brown whereas "Created material 2" it is blank/white when the only difference is the size of the opacity map.
am i missing something? is there a quick fix for this?