On my server, players are cleared of items when they have a leave game score of 1. We also have kitpvp, and if they log out of the game with any armour on, they will still have the armour bars when they relog, even though their inventory is completely empty. Any attributes will also carry through. For example, a movement speed attribute on their boots will result in the attribute still being present when they relog, fully functioning, without actually being on any armour.

This also happened on my old 1.12 server, and it is still very annoying and the only way to fix it is to be killed.