garageCube

I have created my own material and I can see the preview. When I click on update instance / update library a blank/black thumbnail is applied. I can manually replace the thumbnail.jpg with a colourful one in the filesystem and it shows in the library, but if I click update instance / update library it goes back to being blank/black.

How does MM capture the Thumbnail image and why would it not capture what is in the preview window?

Indeed there's a bug, RENDERSIZE was not initialized properly when generating snapshot. We solved that, fix in next release.
However using RENDERSIZE in an ISF makes sense, but in a generator that doesn't mean a lot since when it's used in a surface (quad, triangle...) it will just be 1024x1024, since you might do perspective, MadMapper can't deduce the size in video projector pixels of your surface, it doesn't make sense. We suggest not using RENDERSIZE in materials.
Anyway, thanks for reporting !