You're right, they should be visible but aren't. This remains even after restarting Sweet Home 3D?
You can try to detach/re-attach the 3D pane. Right-click in the 3Dview window and select "Display in separate window". Then right click in the 3DView window again and select "Display in main window". With a little luck the arrows will appear, otherwise the separate window is possibly an option for you.
I have Sweet Home 3D running on several versions of Debian with the Mate desktop and have never lost the arrows. Maybe it's a specific problem with your Linux desktop or desktop theme. Try the default theme and see if the arrows appear.