Yes, it only becomes transparent when the furnace is smelting something.

Is there any errors about it in the log?

Sure！
fml-client-latest.log

Make sure tessellatorReplaceEverything is set to true in falsetweaks.cfg

It's already enabled.

Can you try removing blendtronic as a quick test? It has a furnace texture mod that shows if there's an item in the output slot on the texture, but i forgot to add a config option for it

Try disabling notfine

Even after deleting blendtronic, this still happens.

fml-client-latest.log

I suspect it's an issue caused by betterBlockFaceCulling.

Disabling notfine fixed the display issue. I suspect a bug in notfine's configuration.

toggle {
    # Use more accurate block face culling when building chunk meshes. [default: true]
    B:betterBlockFaceCulling=true
}

Just tested, it seems that betterBlockFaceCulling is not the cause of the problem 🤔