Snapshots feature and purging

I’ve tried the snapshot feature for the first time, just to show a number of color variations of a product, rendered in rendered display mode. However, it does not work it seems. Clicking “restore” has no effect, even though I can see a snapshot thumbnail with the right colors.

Also, when I purge the file in order to reduce the number of materials (many of them unused and too much work to do manually), also the materials that are in the snapshot that is not current are being deleted. It seems, logically, purging should not affect any of the materials saved in a snapshot.