That's effectively what's happening.

The USPS, under DeJoy (who has long been trying to undermine the USPS from within), has posted this site, which attempts to debunk (i.e., gaslight) over what the change is. They claim the same process applies and only is being clarified, but that's false since now postmarks will use the process rather than the sending date and that's a major change. This is key:

While we are not changing our postmarking practices, we have made adjustments to our transportation operations that will result in some mailpieces not arriving at our originating processing facilities on the same day that they are mailed. This means that the date on the postmarks applied at our processing facilities will not necessarily match the date on which the customer’s mailpiece was collected by a letter carrier or dropped off at a retail location. [emphasis added]

This is entirely to help support GOP legal challenges that will claim that laws which allow election-day postmarked mail-in ballots to be counted can be struck down, because they will encompass votes that took place after election day, and therefore selectively invalidate mail-in ballots. It's essentially poisoning the reliability of postmarks to allow for ballot suppression.

Here's a PBS story about it as well. It also notes that the staffing crunches at the USPS and closing processing facilities will exacerbate the issue since "processing" times will increase, especially for high-volume days like election day.

Edit: David Steiner, not DeJoy, is the current postmaster general.