If you have another mod that adds a building capable of running the refurbishment recipe that has innate productivity (for example, the advanced chemical plant from k2so), it'll apply. This is obviously not intended because it's supposed to be just a recycling recipe, not a way to make new packs.

I've implemented a fix myself and it seems to work well. On data-updates.lua, change

localised_name = { "recipe-name.space-train-battery-pack-refurbish-desc" },
results = { { type = "item", name = "space-train-discharged-battery-pack", amount = 1 } },
} })

to

localised_name = { "recipe-name.space-train-battery-pack-refurbish-desc" },
results = { { type = "item", name = "space-train-discharged-battery-pack", amount = 1, ignored_by_productivity = 1 } },
allow_productivity = false,
} })