Since 1.16 the players inventory is glitched.different.

When you log into your character, items like your Assault Boots, Assault Helmet, all guns will have the arrow pointing down so you cant see the inventory of it like the hand rail, buttstock, optics etc.

If you click on the arrows whilst gun is on your shoulder, the arrow will go up.
Place gun back into hands then back onto the shoulder as one does, and the arrow is pointing down again