In the patch notes for update 1.28 it says under the "FIXED" column that the issue with incorrect mouse movement when leaning and using binoculars has been fixed, and then links to this ticket https://feedback.bistudio.com/T174629
My first reaction to this was obviously "woohoo that's a great fix", that is until you get into the game and realise that this "fix" appears to be have been done by removing the option from the game entirely.
In 1.28 it seems like any item that has the "look through" option (binos/rangefinder/NVG/weapon scopes in-hand etc) cannot be looked through at all while leaning anymore, the option simply goes away whenever you press the lean key, and trying to lean after looking down the optic isn't possible either.
Maybe the broken mouse movement was actually fixed and this is just a brand new bug that prevents us from using the optics this way, I'm not sure, but I'm reporting it since it's obviously very misleading/confusing to see this listed as a fix in the patch notes when it was actually removed.