Likely the sliding gear needs to be adjusted, and/or possibly the finger on the chronograph wheel. If you let the chronograph run for a full minute, does the minute counter advance right as the seconds counting hand crosses the 12?
In either case this is not a major issue to resolve, and any competent watchmaker should be able to diagnose and correct this. Much is made of the Cal. 321, but it's a simple movement in the end...
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