I had the same problem, additionally after I fall down, I had the 'throw things in your hand'-bug when crouched
May 10 2016
I can confirm this bug too.
I have a TrackIR 5 without the TrackClipPro.
Usually I need to hit the HotKeys two to three times, before they work.
It is very annoying and with the new fast hitting Zombies I always have to run with axe in Hand when looting. So I'm a sitting duck when charged by an unfriendly Player, since it takes Ages to open the inventory and switch to my gun.