Do you mean like, you load an extra shell into your shotgun?
If so, that's not a bug, because it's just like having a 30 round magazine which you load into the gun. After you load the gun you have 1 bullet in the chamber and 29 in the mag. If you reload at this point you'll have 1 bullet in the chamber and 30 bullets in the new magazine.
Same story with the shotgun, when you cock it, you get 1 in the chamber and then you have space to load 1 more. That's why they refer to shotgun mag sizes as 5+1 or 7+1 or whatever.
Anyway a bug I found with the shotgun was that when you're viewing someone using a shotgun and he starts reloading, the reloading animation gets stuck and the character will keep infinitely spawning the shotgun (while the person playing obviously does not see that and can fire the weapon like normal).