According to the docs the item_swap event is supposed to return:

• player
• main_hand: the item array in the main hand before swapping
• off_hand: the item in the off hand

Note main_hand specifically says "before swapping", yet it seems to return the state after the items would be swapped. Even when you cancel the event. This also applies to the prefilters.

This behavior seems to match the bukkit javadoc (1.9-1.19.3):
https://hub.spigotmc.org/javadocs/bukkit/org/bukkit/event/player/PlayerSwapHandItemsEvent.html

ItemStack getMainHandItem() Gets the item switched to the main hand.
ItemStack getOffHandItem() Gets the item switched to the off hand.

Code to replicate the issue:

bind('item_swap', null, array('main_hand': 'REPEATER'), @event){
	/* This only fires while holding a repeater in the offhand */
	cancel()
	msg(@event['main_hand'])
}

Tested on:

CommandHelper-3.3.5-b309
Purpur-1.19.3-b1895

Thanks. Not sure why I had it labeled backwards. There were a couple other things worth clarifying too.