This was on official server DE 3536.
Apparently, when you swap from this shotgun ( video below ) to loaded crossbow, the bolt and the bolt "safeguard?" are misplaced. It only happened when I swapped it with a shotgun. I haven't tried with other weapons since this was the only firearm I had and couldn't find any other. I did try to swap it with a sledgehammer but it seemed to be working correctly. Also, this only happens when you're in the "standing" pose. Swapping in other poses looks like there's no issue. The problem persists after server restart and whenever I log back in. Also, when holding a crossbow in your hand, and relogging, the next time you log back in, it will look as if it's empty , but there's still a bolt inside. And swapping to a different weapon won't fix it, and will make a "click" sound during the swap ( same as the one on offline mode - explained in the paragraph below ). You have to shoot the crossbow and then I guess it resets itself to a "empty crossbow" animation state..
I've tried to replicate it in the Community Offline mode, but I can't since there's a different issue in offline mode, where whenever you swap a loaded crossbow with a different weapon, it will make a clicking sound ( like you've pulled the trigger on an empty crossbow ) and when it's on your back it will have the bolt in the crossbow but the string will be released... and then, when you swap again to your loaded crossbow, it will make the same clicking/shooting sound of "shooting an empty crossbow" and it will look completely unloaded ( with the released string ). But obviously, if you shoot, the bolt will come out. So it's just a visual bug as well..
EDIT: I've managed to test the bug with two more different firearms on the same server ( DE 3536 ), and the visual bug persists. It was savannah, and double barrel. So it might be connected to all large weapons ? because pistols "fix" the animation state issue, and seem to work properly just like sledgehammer does..
note: I don't think it's related to pulling a weapon from the shoulder, because I've tested it by pulling a sledge from my shoulder and it seemed to work. Unless it's a combination of Firearm and it being on the shoulder slot.