Why is TurboTax telling me I need to make only 3 estimated state tax payments for 2021?

Just completed my 2020 taxes, but I find it odd that TurboTax is telling me that I need to make only three estimated state tax payments for 2021.  TurboTax says I need to make payments on 4/15/2021, 6/15/2021, and 1/15/2022, but not on 9/15/2021.  My state is California.  I've never seen this before.  Is this a bug?  

 

For Federal estimated tax payments, TurboTax says I need to make the usual four payments, as expected.