Rear Audio Missing When Using 5.1 / 7.1 Surround – 3D Headphone Works Correctly

Platform: PC

Issue:
When I select 5.1 or 7.1 surround sound in the game, I cannot hear audio coming from behind my character.

The rear/behind audio appears to be completely missing or extremely difficult to hear.

However, when I change the audio output to 3D Headphone, the directional audio works correctly, including sounds coming from behind.