Yes, deleting an order does not automatically delete and deactivate the tickets that were created for the customer. This means that even if the order was refunded or deleted the customer will still be able to use the ticket i.e. when scanning the ticket barcode using the Event Check-ins app the ticket will still show up as valid. You will need to refund the order and then delete the ticket and update the stock number manually if you are managing this through WooCommerce. This is in-line with the standard WooCommerce functionality, where you might want to refund someone but still let them keep/use stock.
If you are using the FooEvents Seating functionality then you will also have to permanently delete the actual ticket associated with the order before the seat will become available again. Alternatively you can also make the seat available again manually.
Here are instructions on how to achieve this: https://help.fooevents.com/docs/fooevents-seating/modifying-seats/