Why do convex curves with sharp edges automatically flatten? I've been making a few weapons for a short while now and I've found that a similar issue arises pretty commonly. This time I've noticed that when making a convex portion of a blade with an edge that sharpens to nothing, the curvature is lost and somehow flattened prematurely. 

The blade is two-sided but there should be no clipping faces, I've already merged vertices and tried remaking the faces. I know that making more edges to join the outer and the inner vertices can resolve this, but it doesn't preserve the smooth look of the blade. I also noticed that this problem doesn't arise when only one side of the sword has the face that's causing the issue.
This is blender 2.8
This happens because the surface is pretty much undefined. The only guaranteed flat surface there is is between exactly three vertices. If you have more vertices without edges,  it's a guessing game for Blender, so it will come to different conclusions, depending on angle and other factors.

What you can try is using the subdivision surface modifier, simple, not cattmull-clark, in a high resolution. It will try to fill the face with edges.