This is a funny situation that the IRS doesn't fully address. When you receive a 1099-SA for this distribution, just report it in the year to which the form belongs (not the original expense). So if you do the reimbursement now, you should get in early 2020 a 1099-SA reporting it, and you will enter this on your 2019 tax return (which you will be doing in early 2020).
