My understanding was they don't pay interest on tax refunds, but you pay interest on unpaid tax liabilities.

Do you have a source handy? My Google-fu is failing and only bringing up interest on unpaid taxes.

If the refund is paid in less than 45 days after the tax filing deadline or the day you file your taxes, they will pay you interest. See here: https://www.irs.gov/payments/interest#pay

Within the last few years I got a couple hundred bonus bucks back from the IRS as interest. Might've been a year where they were overhelmed and took significantly longer to cut the check that usual