Strange, you might have a corrupted project file but try if this fixes the issue:

First make a backup copy of your .sh3d project file.

Open your original project.
In the Preferences dialog (Menu > File > Preferences) there is an item "Furniture icons in plan". Switch that to the other setting, save your project and close it.
Reopen the project and switch back the setting in the Preferences dialog and see if it is fixed.