When smooth GUI is turned off, the progress bar in furnace GUI stops rendering completely. Might be an issue with just resource packs, but pack works normally without Hammer Lib. Turning the option on only renders the background of the arrow when a resource pack is loaded and ignores the resource pack.
To replicate, load a resource pack that changes the furnace GUI, turn the option off and smelt items.

HammerLib-1.12.2-2.0.6.32