Thanks for the quick response.
It should be about as planar as floating-point precision allows. I created an icosahedron using add->icosphere, then added and applied a bevel modifier (corners only).
You could try selecting the face and set it to smooth shading.
Select all faces and use Ctrl+N to recalculate normals.
Neither of these seemed to solve the problem.
This face will actually be two four sided faces, set to be an FGon. You can show the real faces by selecting the face, pressing F and select Clear FGon
You say it’s causing problems. What kind of problems?
I use the face normals programmatically in my game to position/orient the camera over its surface. Each face works as expected, except for this one, which produces an incorrect orientation. FGons are also important, as my custom export script converts them to NGons to simply some calculations in-game.
As you haven’t attached your blend file this is just best guesses
Blend file attached.
trunc_iso.blend (126 KB)