The 3rd person crosshair and ADS sighting don't stay congrous with each other when prone. When switching from 3rd person to ADS (while prone), the aim declines by between 20 and 30 degrees, resulting in the player aiming down into the ground. If the player corrects their aim upward while in ADS this correction is then translated into a 20-30 degree incline in 3rd person aiming instead, causing the player's 3rd person crosshair (and weapon) to aim significantly upwards.

This can be corrected by aiming straight down in 3rd person while holding LT, in a crouched or standing position, which appears to realign the crosshair and weapon muzzle again.