I have a sense that this could be a problem.

On the evening of April 18, I entered a total of four Q1 estimated tax payments, across the three processors. All receipts confirm April 18 payment date.

Looking at the IRS EFTPS site, all show Settlement Dates days later, which was expected. However, they all show Initiation Dates ranging April 19-21.

I hope I won't have to fight a late payment penalty later. Ugh.