Boolean Union makes one object dissappear

This has happened a couple time before. I checked and both objects are valid Rhino Poly surfaces. When I Boolean Union the second object disappears after the process.

BooleanU.3dm (1.2 MB)

file, or it didnt happen.

The one that stays is an open polysurface with its normals facing the opposite direction of what you expect. Because of that, the one that disappears is actually completely inside of the other one, so it would not show up as part of the union. Call Flip on the one that stays and you’ll get what you expect when you call union.

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How do I know it is flipped? Should an open polysurface be able to union? If not, how do I find what’s open to make it closed.

You can check and edit normal direction using Dir command.
You can also changes the color of backfaces (the side opposite of the surface normal direction) from rhino options:

What and List helps you to find such information about selected objects.
You can also use SelOpenPolysrf to select all open polysurfaces.

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as @Mahdiyar says, you can configure the view mode to always show the backfaces with a different color to quickly see which object is flipped. also you can configure to always see naked edges: