I bet I know what happened. You were eligible for 4200 (1400x3). You probably got one letter 6475 saying you got 2100 for the 3rd Stimulus payment. So you entered 2100. And it gave you the remaining 2100 on line 30. But your letter was only for your half. You spouse also got a letter for the other 2100. So you needed to enter the total you got - 4200. Go back and check your bank account. I'm sure you'll spot it.
