Your original post is confusing ... so you had an FSA in 2018 and you asked for a reimbursement for a 2018 bill in early 2019 (before the cut off date allowed) and they sent you the money in 2019. If this is correct then there is nothing to report on an income tax return ... it is NOT income on the 2019 return. And what you use the money for is immaterial.
Now if you has an FSA for 2018 and you asked to be reimbursed for a 2019 expense then that is also ok if the FSA plan allows you to do so up to a cut off date which is usually sometime in March of the following year. Although it is possible for the FSA administrator to make a mistake it is rare.
