http://feedback.arma3.com/view.php?id=6389
This was a previous post regarding the issue but the cause was incorrect, so I investigated the issue a bit more.

The problem occurs when using addweaponcargo - when more than one of a gun is added to the container the weapons adjustable sights no longer work.
For example:
this addweaponcargo["srifle_EBR_F",1];
Will work fine.
While:
this addweaponcargo["srifle_EBR_F",2];
The range adjustment hud will be missing and page up and down will not do anything.

I've attached a repo mission and I have it working consistently, one crate only has one EBR the other has two. {F18584}