You can confirm this ticket https://feedback.bistudio.com/T93397#1831735 , as it is very old, but is being reproduced now.
This is not a proper use of the ACOG scope features.
This part of the sight is not activated on any rifle.
On the M4, for example, where the iron sight is removed to install the ACOG optics, only the ACOG optics work, and the switch to the ACOG iron sight does not occur.
On sg5k with ACOG, switching to an iron sight shifts its eye to the sg5k iron sight, which leads to a poor view of the target.