If you *think* you may have an under tax withholding penalty if/when that $5K FSA money is taxed, you might want to go ahead *right now* and pay tax on that money. You can pay online at www.irs.gov/payments.
Basically, if at tax filing time the amount you owe the IRS is more than $1000 or more than 10% of your total tax liabiillty (whichever is *higher) then in addition to paying those taxes, you'll be assessed an underpayment/under withholding penalty. It all really depends on how much was withheld from each of your paychecks in 2019, which was based on the W-4 you filed with your employer at the beginning of the year. So if necessary you can go ahead and pay the IRS say, $500 "RIGHT NOW" and if it turns out you've overpaid, it will be refunded to you when you actually file the return.
The above also holds true for your state, if your state taxes personal income.
