Erratic behaviour when trying to create a solid

currang2

In 2017 Pro I created a non-prismatic but relatively simple volume made up of 4 sided non-parallel shapes, but it wasn't recognised as a solid. All surface were oriented the correct way.

I then broke the surfaces into triangles. Still not recognised as a solid.

Then I deleted the diagonals to get back to the original shape made up of 4 sided non-parallel shapes. Suddenly it was recognised as a solid.

Why. I can't see a logic in this experience.

I have experienced similar behaviour in the past and gave up on trying to create a solid after spending a lot of time trying to edit the shape as I had tried this time.

Is this a bug, and can it be fixed or worked around?

This is the model referred to in the message.

Box

It's a bit hard to tell you anything when you give us the solid version, could you try one that isn't showing as solid.
Technically even this version isn't a solid,it should really be defined and a void surrounded by an infinite nothingness. This is because you have the faces reversed, meaning the shape is inside-out. When viewed in Monochrome you should only see white faces, the blue/grey faces are the back or internal faces.