There is an interesting TurboTax question, and that person says their Letter 6419 is wrong.

They looked at BOTH of their online IRS accounts, and the IRS accounts match the amount that they actually received.  But the combined Letters 6419 is showing a different amount.

It is possible it is user error somewhere, but the fact their online IRS account matches the actual amount they received makes it seem legitimate.

Sigh.  It may be another long tax season.