I just found out it has something to do with reloading with holding R and also slug ammo turns into buckshot ammo visually but when you reload with buckshot all your slug ammo comes back visually. It is a very weird bug. If you use all your slug ammo doing it which makes it so it completely disappear from your inventory you exit the server and come back to find all your slug ammo on the floor or buckshot I think it has something to do with what was the last ammo you reloaded. Definitely weird and complicated unless you try it yourself.

This problem was super rare for me but my friend who play on the same server (official) had this issue everytime.