This is typically caused by duplicating a product discounted by the app. Copying products discounted by the app should be avoided. This will cause the app not being able to undiscount the product and your store will think it's still on sale.Consequently the price will stay at the discount amount.
To fix this, start a discount with that product selected. A small discount (1%) is all that is needed. Allow the discount to start. Once complete, stop it. This will remove the sales icon. If the item is still discounted then you will have to manually change the product price back to what it was presale.