Why do barrels on harpoons despawn?

For context, me and my mate sink a sloop in HG, we start harpooning and i Harpoon two barrles on two different harpoons to realize it's stacked, i go to grab a storage crate and right then a server merge happens, the rest of the barrels (only having coconuts bananas and fireworks) stay in the water but the ones on the harpoon despawn, why can the game keep up with the stuff in the water but not on a harpoon?