That's exactly what happened to me.For future reference, I recently found out that not all mobos 19 pin usb 3 front panel header has same orientation so check carefully which side is missing the 1 pin because that corresponding position on usb 3 header plug from front panel will not have a hole for pin to insert & trying to forcefully insert into wrong orientation will break that pin.
The header is side-mounted on my mobo and the pins are hard to see. While inserting the connector, the orientation was slightly off and the pins bent.