Nothing Burger

This has been a thing for at least 10 years, and you can see articles going back that far about it for timely local tax payments sent by mail.

The "postmark" exists to cancel postage (that is, to mark the stamp so it's no longer usable). In the modern postal service, that gets done by the automated sorting machines. So the postal worker collects the mail for the day, and then ships it off to the sort facility. It may get a postmark applied that same day, or it could take 1 or 2 business days before your mail gets into the sort machine, and then you'll get a postmark that's a few days later.

If the postmark date really matters to you, like for timely tax payments and mail-in ballots, the procedure is to go see a clerk at the post office and ask for them to apply a postmark right then. You'll see it physically applied and with the current date. So any recipient will know your exact date of mailing.

Even with the manually-applied postmark, the machine may apply an additional one as well with a later date. But recipients should look at whichever date is earlier.

This is an efficiency thing and has been in place for at least 10 years if not longer. To actually ensure every piece of mail gets a postmark on the day it was collected increases manpower and costs, for something that usually doesn't matter for 99% of mail.