Part of this simple model is the most convoluted example of the long-standing “bow-tie” bug I have ever encountered! I haven’t a clue how it was generated.
The skinny triangular face at the -red side is ok (one of its edges is soft and smooth, so you see it only if view hidden is on).
The rest makes up a tangled mess! It all is captured as a single face with normal vector +z in the model database. All the vertices of the face are at z=4490 exactly; non-planarity is not an issue.
The face starts its outer loop by following three sides of the skinny rectangle at the top, starting at vertex [~4961, ~6397, 4490], going from there to the upper right vertex at [~5609, ~6517, 4490] and thence to [~5605, 6510, 4490] and then [~4957,~6389,4490]. Because this loop is left-hand order with respect to the normal (we’ll see how it gets closed later), SketchUp’s rendering and selection logic treats it as a “hole” and does not fill it.
The loop then goes down the edge from that corner to the top of the large triangle at the bottom at [~4991, ~6260, 4490]. The loop then traces the outline of the large triangle in right-hand order, going down to [~5070, ~5928, 4490] → [~5311, ~5984, 4490] and then back to [~4991, ~6260, 4490]., closing the large triangle.
Then it retraces [~4991, ~6260, 4490]->[~4957, ~6389, 4490] in the reverse direction from previously!. Finally, it completes the loop by returning to [~4961, ~6397, 4490].
So, we have the double oddities of a left-handed loop at the top and an edge that is used twice in opposite directions by a single face!
Evidently all this weird geometry causes the problem with moving vertices in the z direction until some other edit is performed that causes SketchUp to “heal” the defective face.
