If you put on some equipment (e.g. vests, backpacks, clothes, head gear) out of an arsenal container (right mouse button or directly drag it into the equipment slot or drag it over the avatar) the inventory user interface will not get updated. At the same time you can't discard the equipment items you put on as long as you keep the inventory interface open.

But you can add magazines, medical items, grenades etc. (with right click or directly dragging and droppin on your avatar).

You have to close and re-open the inventory user interface to see the changes in the user interface and to interact with them (discard items).