I often use appearances in my models, to make it easier to differentiate between the parts in th assembly, but it also makes it look better, when showing images to the customer.
But now I am experiencing something strange:

When I open the assembly everything seems fine. But sometimes, after doing a rebuild, all appearances disappear, leaving me with a gray mesh of parts.

When I close the model and opens it again, everything is back to normal. Until it happens again.
I attached an image to show what I mean.

Can someone help me?