Using mouse-middle as to move an item into the hotbar is having no effect

[BlvckBytes]

'/viaversion dump' Output

https://dump.viaversion.com/raw/03611d1b595690f9d6c5b60e6587979f695f766507ddde47aa874b5cc57700d7

Server/Client Error

No warnings or errors of any sort have been encountered.

Bug Description

Moving items from the inventory into the hotbar by klicking mouse-middle does not work on versions other than the server's native - 1.21.4, in this case. The expected behavior would be that the item-stack corresponding to the currently looked-at block is moved into a vacant slot within said hotbar, and that the currently selected slot is changed to it; works flawlessly in singleplayer-mode.

Steps to Reproduce

1. Set up a server with the latest version, namely 1.21.4
2. Install the latest ViaVersion as well as ViaBackwards
3. Join with a client that's not native to the server's version, so that protocol-translation has to occur
4. Break a block - dirt is often most suitable
5. Move the block up into any inventory-slot other than the last row, as to cause it to be out of sight relative to the player's hotbar
6. Press mouse-middle on a dirt-block
7. Observe how the item is not being moved down

Expected Behavior

Repeat the steps mentioned above on a local singleplayer-instance; the feature now works as expected. The same holds true on a client that's native to the server's version.

Additional Server Info

No response

Checklist

• [X] Via plugins are only running on EITHER the backend servers (e.g. Paper) OR the proxy (e.g. Velocity), not on both.
• [X] I have included a ViaVersion dump.
• [X] If applicable, I have included a paste (not a screenshot) of the error.
• [X] I have tried a build from https://ci.viaversion.com/ and the issue still persists.