Minecraft version: 1.20.1
Modloader: Forge
Forge version: 47.0.35
Environment: Multiplayer

Mod name: Stackrefill
Mod version: 1.20.1 - 4.0

Hey! I have discovered a bug with the mentioned version.

This also occurs only once I am on our server.
In singleplayer the mod works just fine.

We have a paper server, which loads the mods in through Forge.

Other mods we use on our server are:

Xaeros-Minimap-Mod-Forge-1.20.1
World-Map-Mod-Forge-1.20.1
worldedit-mod-7.2.15
OptiFine_1.20.1_HD_U_I5

and of course for stackrefill to work
collective-1.20.1-6.72

Also, I have already discovered an almost similar one here in the open tickets, but the person has not answered further.
The mentioned ticket was from March 26 this year from user: DaisyDumm.

Here's what happens:
When I place the last few items of a stack (while I have the rest of the stack in inventory), nothing is placed in the hotbar from inventory and the stacks in inventory are invisible.
As soon as I click on the place where the invisible items are, they become visible again.

I have also recorded a short video to better illustrate the bug.
The first scene is from my singleplayer world, the second from our server.

-> https://youtu.be/ClzwojncO9g

I would be interested to know if it is a general bug or if there are problems on the server side.

I can confirm

Can confirm it too, as showed in the video

Same here on 1.20.6 with the following mods. Singleplayer works but Multiplayer doesn't:

We have a paper server, which loads the mods in through Forge.

I'm a little confused by this part. Do you have a Paper server and Forge clients, or some kind of Paper+Forge hybrid server?
Is stackrefill installed only on the server, only on the clients or both?