Some unions, while appearing normal (including when exported as an OBJ and viewed in another program like Blender) have incorrect geometry when using raycasting which results in a number of weird things occurring. Here’s an example of a grid of raycasts from the camera’s POV to the baseplate:
Green dots are raycasts that hit something other than the baseplate. There are a number of green dots on areas where no parts exist, and mousing over these areas in studio shows that the engine thinks the parts are actually in those locations:
As shown in the video above, one of the side effects is the inability to select those unions easily, but it appears whether or not a part is rendered is also based on this incorrect data. Moving the camera can sometimes cause unions to disappear (I think this bug report is caused by this issue) when the engine thinks they have gone off screen based on the false data:
The shadows still render but not the unions themselves.
Separating and re-doing the union doesn’t have any effect. Changing the union size doesn’t fix the issue either. The unions display correctly (minus the disappearing), textures/decals on the unions render correctly, and exporting the union to an OBJ works fine.
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