I noticed that you sometimes can have two stacks of the same ammo type.
This happens because all things attached to a weapon follows when moved to a storage box at a safe house. (Which is a good thing). But since the ammo is (kind of) attached it won’t stack.
The big problem appears when you gather more ammo of the same type out in the world and decide to bring back your weapon from storage. What happens is that you also get two stacks in your inventory, but only one stack is attached (the one retrieved from storage).
In the pictures I have 6 shells attached to the shotgun and another stack of 30 of the same type. When looking at the HUD everything looks fine.
But after firing the six shells attached this happens:
I am not able to fire the remaning 30 shells and the HUD displays “00/0000”. When entering the inventory and attachments the other stack is attached, but in reality it is not.
To get it to work you have to unattach and reattach the shells. Nothing game breaking but I thought it could be worth your attention. 
