Was it IRS letter 6475? That is to tell you how much they sent you for the 3rd Stimulus. Not just how much you are eligible for. People who did not get the 3rd Stimulus payment did not get a letter.
Was it this letter? This blog shows both letters for 6419 Child Tax Credit and 6475 Stimulus
https://blog.turbotax.intuit.com/tax-planning-2/irs-letters-6419-and-6475-for-the-advanced-child-tax...
