Exactly what you did, write another dump to it and adjust the mileage. Can be caused by corrosion, or jump starting it with a battery charger on boost when the batter is dead.
One of the ones I repaired the mileage string was fine as well, but the dump was corrupted elsewhere. Another one the mileage was corrupted and editing it brought it back.
Another one from a Trailblazer had 150,000 Km on it, but after they boosted it with a battery charger it read 760,000.
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