Yeah, it's a relatively new route and runs up through State College and towards Williamsport. It "officially" ends in between those two places, but then if you continue north from W'port on US 15 towards Corning, as soon as you cross over the PA/NY border, it becomes I-99 again...odd.
Wikipedia says "Unlike most Interstate Highway numbers, which were assigned by the American Association of State Highway and Transportation Officials (AASHTO) to fit into a grid, I-99's number was written into Section 332 of the National Highway System Designation Act of 1995 by Bud Shuster, then-chair of the U.S. House Committee on Transportation and Infrastructure, the bill's sponsor, and the representative of the district through which the highway runs. I-99 violates the AASHTO numbering convention associated with Interstate Highways, since it lies east of I-79 but west of I-81."
Here's the Wiki map...
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