I'll just pop in really quickly here. It's a feature not a bug, essentially the longer the you hold the trigger the more recoil gun amass and becomes more difficult to control therefore sensitivity lowers.
The idea behind it is that yes you can do very long bursts with certain weapons but it also has a price in accuracy and lowered ease of use. Short controlled bursts are rewarded with more accuracy at the cost of time to kill (which you can compensate by headshots). It is also set per weapon variant so maybe it's just specific variant that is too difficult.
I'll add looking at this to my list... but it's a long list at this moment.