As you can see in the video, after shooting the FEL-IX and you try to switch weapons (by pressing 2) it starts the c*cking animation before the weapon switching starts. that is probaly also the problem why you need to press 2 two times to change weapons after firing your last bullet.
the video is made of 5 clips
the 2 chapel clips is me pressing 2 and releasing the ads button after shooting, and then pressing 2 while ads’ing. in the slomo you see a bit of the c*cking action of the sniper before the switching starts.
to make sure its a bug and doesnt happen because i scope out at the same time as i switch weapons i made the terminal clips.
3rd clip (first terminal clip) is me firing the FEL-IX without releasing the ADS button and pressing 2 after firing, as you can see there still is a delay.
4th and 5th clip are showing me when i press 2 after firing the last bullet of the magazine. as you can see it still doesnt switch immediatly, and you have to press 2 twice before weapon switching actually starts.
TL;DR: there is a delay when switching weapons after firing the FEL-IX while there is still ammo left in the magazine, if there is no ammo left after shooting the first keypress to switch weapons is ignored.
@FrostyVampire has also mentioned this problem, idk if she made a post about it here yet.