Since, besides your arena plugin, there are other arenas and PvP zones, and entering or leaving the arena happens far less frequently than taking damage, I believe it would be better to call universal hooks that my plugin is subscribed to. Besides, I have already tested them with your plugin, and everything works perfectly if you add the hooks in the right place. If you’d like, I can tell you exactly where.
OnPlayerEnterPVP - is triggered when a player enters a PvP zone.
OnEntityEnterPVP - is triggered when an entity enters a PvP zone. In your case, it's only DroppedItemContainer(the bag after death).
OnPlayerExitPVP - is triggered when a player leaves a PvP zone.
OnEntityEnterPVP - is triggered when an entity leaves a PvP zone.
More details can be found on the plugin page in the Developer API section(Almost at the very bottom of the page).
Thanks in advance!