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?