When playing on a 1.16.5 paper server, if the client has this mod installed the inventory crafting (2x2) never shows what it is that you are crafting on the right hand side, but will still allow you to click and collect the crafted item.

Is the mod present on the server? I have no idea what paper is.

No, the mod is not on the server. Paper is a lightweight/highly optimised version of Bukkit and Spigot

This mod must be on both sides to function properly.