The entire testing area is a shopping area, so you can never lose the 'grace period'.
I tested in a live match and the grace period is lost the moment you leave the shop, so yes it is a bug. Reloading with an item that increases the amount of ammo you have should force to have to sell the item.
Also, it's worth mentioning the bug only gives you one extra bullet.
View attachment 4465
Yes it shows as way more, but when you actually shoot, you only have one extra bullet. Your clip shows this as well, it does not give you anymore than that.