Why has stock been deallocated from an order and allocated to another and I can't find the original order?

This is perfectly normal. This means that you have import unpaid orders switched on on more or more of your channels. This is to reduce oversells to "reserve" the stock.

This will happen if you have "Import unpaid orders" enabled on your integrations. Orders will import like this as the unpaid order will import when it is initially made and then deleted, so the paid order can be imported when the payment is processed and approved.

Once the paid order comes in, the stock is deallocated from the unpaid order and allocated to the new paid order. The old order is then deleted.