New toilet runs on after flushing

Hi everyone.
I have a 6 month old Briggs Vacuity toilet. Recently it's started running on after flushing. After it's flushed, the bowl fills up with water until the water is about 4 inches from the top, then it stops running and the water level in the bowl drains slowly until it reaches the correct level. Why is the bowl filling up too much only to drain right out to the proper level immediately after?
Any help is appreciated.
Brian
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