Really straight-forward issue. We had a bug like this (iirc) when Wendigo was first released.
Basically, if you enter stealth, you have no visible iron sight, so you can't aim down sights effectively.
A really simple fix for this would be to allow the tiny luminescent tip of the forward sight to be visible while in stealth.
Here's an example, it's SUPER quick and dirty. Just an overlay with some color tweaks:
https://gyazo.com/749ce37507830ebdf7a66bb13cfbb906
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