merge-radius not working as expected

Expected behavior

Using spigot's config option: merge-radius: items:

Throwing an item towards a stack of the same item on the floor will combine the single item with the stack.
The existing stack of Items will be pulled to the position of the newly dropped item.

Observed/Actual behavior

Now in paper versions >=1.21.5 the new item will be pulled to the existing stack in the world, thus rendering most sugarcane and bamboo farms useless.

Steps/models to reproduce

Change merge-radius: items: -1 to merge-radius: items: 4 in spigot.yml

Plugin and Datapack List

Plugins: PlugManX, *SecondThreadAPI
Datapacks: [vanilla (built-in)], [file/bukkit (world)], [paper (built-in)]

Paper version

This server is running Paper version 1.21.8-60-main@29c8822 (2025-09-06T21:49:13Z) (Implementing API version 1.21.8-R0.1-SNAPSHOT)

Other

I narrowed down the issue to be paper specific, as spigot 1.21.8 works as expected.

Start by reproducing the behavior with merge-radius: items: 4 in spigot.yml on Paper 1.21.8, then compare it with Spigot 1.21.8 using the listed reproduction steps. Done means the existing item stack is pulled to the newly dropped item and the items merge as described, without the listed plugins or datapacks changing the result.