Reproduction steps:
1. Open the attached project "FrustumTest.zip";
2 Observe the gizmos drawn in the scene view.

Expected results: GameObject "Near" is at the plane's top and the "Apex" is above the plane.
Actual results: "Near" is inside the plane and "Apex" is drawn at the top of the demonstration plane.

Reproduced on: 5.6.6f1, 2017.1.4p2, 2017.2.3p2, 2017.4.6f1, 2018.1.7f1, 2018.2.0b10, 2018.3.0a3.
Not reproduced on: 5.4.6f3, 5.5.6f1.