The reason why the inventory for the products within your kit decreased or went out of stock when you created a kit is that the kit itself does not have its own separate inventory. Instead, the kit draws its inventory from the main products included within it. Consequently, when orders or FBA shipments are associated with the product that is now converted into a kit, the inventory is deducted from all the main products within the kit.
A similar scenario occurs when you map existing products as main and subs. In this case, the historical data from the sub products’ FBA shipments and orders is subtracted from the inventory of the main product.