How very interesting!! You are correct, it kept the 5 digit sequence on your numbering when it "rolled over". The PO Number is controlled by the "Last PO Number" field in the PO Setup screen in each application database. Note you only get one per database - so if you have multi-company in the same database as we do - you can't have sequential PO numbers by company. You must have started up the other application databases at a different point and using a different PO Number sequence which does allow you to have nice sequential PO numbers by company.
I did test (in our test database) changing the "Last PO Number" in the PO Setup screen from 6 to 4 digits then to 5 digits then to 6 digits and it worked fine. So all you need to do is to just update your "Last PO Number" on your setup screen to fix it.
But I didn't know it did that. Thanks for the question!
Gail J-N