Help. Masons are Gone

I had/have 2 masons working next to each other. Now their building are gone. I can't tell they are there unless I put curser right on their last location. Then I see the shield with their lvl (2) but no building. I will try to reboot and see what happens. They still seem to still be working in their stone pits.

Reboot worked. Buildings are back. But where did they go and why?

It's just a visual bug -- every now and then a particular building's appearance might not load, so while the building will be there and will be working you might not see it. Reloading will generally fix that when it happens.