In general, for 3D tetrahedron plot:

• If 3 of 4 components = zero, you land on a vertex of the tetrahedron
• If 2 of 4 components = zero, you end up on a side line of the tetrahedron
• If 1 of 4 components = zero, you land on a side surface of the tetrahedron

but for now, when Z = 0 (column C = 0), the data point wasn't land on a side surface.

Bug in Origin 2019 since this feature was introduced. Fixed in Origin 2019b.