Why would I have an underpayment penalty for state taxes if I am owed a refund

[baldietax]

if you didn't meet the quarterly timely payment requirement, you can still have an underpayment situation for earlier quarters, even if you met the safe harbor for the year or even had a refund.  you'd need to check the details for your state to see when/why the underpayment happened.

[CesarJ]

If you don't withhold or pay enough tax on income received during each quarter. Even if you paid your tax bill in full by the deadline or are getting a refund, you may still get an underpayment penalty.

@diamondlight