It seems that empty barrels in the glass panel recipe scale with productivity, which results in furnaces with a built-in productivity bonus producing more empty barrels that the full ones they consume. They should probably get an "ignored_by_productivity" value added to them.
In a way this is just a compatibility issue, since it's not a problem with only Arig installed, but it's probably easier to fix it here rather than blacklisting the recipe in every mod that adds a furnace with a productivity bonus.
