I don't know if this is what happened but it could have been put in the wrong PO box and the person that got it kept it instead of turning it in.
Fortunately most of the people at our post office are really good but a couple of years ago I had a card that showed it was delivered to our PO box but it wasn't there so I went in and told them I was supposed to have a package and when they checked it had been put in the box below ours by a substitute worker.
I also noticed lately that some tracking says that it was picked up as soon as it's put in the box.
Here's an example of that on a recent card I purchased I didn't pick it up until the afternoon but the tracking says "individual picked up at PO" at 9:02am but that's when they scanned it and put it in the PO box.
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