Hello there.

In the 1.21 patch notes there is a fix listed for the issue that would cause your aim to be misaligned while prone on an incline "Aiming while prone would not take the terrain surface into account normally" and from what I can tell, it seems that this half a decade old issue has finally been resolved! This is fantastic news, thank you for that.

However, this fix only applies to firearms, the issue still remains when using something such as binoculars or a hunting scope.

Here you can see the difference between the M4A1 and binoculars. The M4 now aims at the crosshair as intended, but the binoculars still aim below the crosshair when laying downhill, and above the crosshair when laying uphill, just like before.