I’ve seen single coil pickups with rod Alnico magnets that had opposite magnetic polarities from pole to pole. Either someone rewound the pickups and accidentally put the poles in upside down or the poles were magnetized first then pressed into the flatwork. Having magnetized magnets while assembling the bobbin can cause several problems. It can cause phasing problems from pickup to pickup and within a single pickup. Pre-magnetized rod pole pieces can pickup up debris especially when grinding the magnet to put a chamfer on one end.