It may be a bit of both, but to give you an idea how the current inventory works, is that this is apparently inherited from legacy code. The inventory system still uses design from FFXI (2002) since the game was originally trying to repeat the success of that title. The inventory system has created quite a bit of
technical debt. What the game does in order to prevent as little loss of your items as possible (as well as cheat prevention) is that creates a capture point/transfer of all your inventory every 15 seconds.
It does this for example, every time you zone or queue into duty.