Note: In order to access the kettle interface, I needed to use Adlumia7's v3.0.3 of the mod, but as that fix doesn't change how the kettle interface works (just how it's opened) it shouldn't be the cause.

If a player puts 2+ coffee beans in the top or bottom inputs, then regardless what is put in the other slot coffee will brew, allowing coffee to be crafted with a single coffee bean and any other item.

For example, if inputs are [ coffee bean x2 ] [ dirt block ], then coffee will brew and inputs will become [ coffee bean ] [ nothing ].