When you craft a stack of capacitor backpacks with a chestplate, the stack is stored in the chestplate's nbt and one capacitor backpack is removed from the crafting window. If you then craft the chestplate to get the capacitor backpack back, you get the whole stack.

Link for easy reference:

IE version 1.16.5-4.2.4-134
Forge version 36.1.4

I tested it with just Forge and IE, but you will see it clearly in the code. Who crafts with stacks of the things anyway?

For a bit of backstory, I was originally testing what worked with the capacitor backpack and what didn't in a mod pack that includes Simply Jetpacks 2, which it turns out just voids the capacitor backpack when combined, so I added the following to the IE server config blacklist.

"simplyjetpacks:jetpack_creative", "simplyjetpacks:jetpack_creative_armored", "simplyjetpacks:jetpack_ie1", "simplyjetpacks:jetpack_ie1_armored", "simplyjetpacks:jetpack_ie2", "simplyjetpacks:jetpack_ie2_armored", "simplyjetpacks:jetpack_ie3", "simplyjetpacks:jetpack_ie3_armored"

Not sure if that is the sort of thing you might want to add to the IE config or not. Either way, thanks for the awesome mod.