I have a golden furnace, from the mod Iron Furnaces. It ejects ingots into a drawer controller linked to 3 drawers above it. Controller accepts ingots and voids them instead of adding them into correct drawers. Had the same issue with a powered furnace from FTB Industrial Contraptions.

Which mod version are you using

1.18.2-1.0.6
Also, prior to locking the top drawer, the controller stuffed each empty slot with 64 iron ingots and then stopped filling them.

I think the furnaces are checking if the slot has 64 items and doing strange stuff, they shouldn't be doing that as the drawers can hold more than 64 items per slot