Platform Builder 10.5.2 has a number of fixes. Most importantly, a system has been added to help restore missing games automatically. Some have reported this problem, and so here I am at 1 a.m. with my fingers crossed that this will fix the problem.
I suspect this issue originated with the Steam Cloud feature that was never meant to be activated for Platform Builder. I don't know how it switched on because nothing was configured properly, but I turned it back off.
10.5.2
-Added a function to help restore missing game files automatically.
-Built-in star items now use the levitate setting.
-Added a volume control for imported music (technically, we already had it, but it was not visible for a while).
-Fixed: The manager button appears when defeat settings for custom enemies is set to "Do: Nothing."
-Fixed: The assistant is not visible when speaking in Game Setup.
-Fixed: Right-clicking on edit button in the workstation doesn't work if the button is disabled.
-Fixed: The "select" button in enemy settings does not work for when the enemy defeat settings is set to "New Area".
-Fixed: The wood tile set displays the wrong text in the workstation when it is selected.
-Fixed: The wrong message appears when you are asked to start a new project in simple or advanced mode.
-Fixed: Delay physic setting for custom projectile does not save correctly.