I know of two ways for this to happen, but there are probably more.

If an offer was previously an FBM offer and was converted to FBA then if it had stock before it was converted, the system can send the orders through to you when FBA is sold out.

If you have a third party app or an integration for stock management (e.g Veeqo) that has connected your FBA offer to the same SKU in its inventory management as the FBM SKU. This is effectively the same thing going wrong in that the Amazon system has a record of that SKU having FBM stock against it, even though it's an FBA offer.