The reason why the inventory for the products within your kit went down or out of stock when you created a kit, is that a kit does not have its own inventory; it is pulling the inventory from the mains that are in the kit. This causes the inventory of all orders or FBA shipments of the product that is now being converted to a kit to be deducted from all the mains within the kit.

The same thing can happen when you map existing products as main and subs. The history from the sub’s FBA shipments and orders is being deducted from the main inventory.