How do we correct a ‘defective’ performance improvement plan?

Q: “Is a difference in date a valid reason to have a performance improvement plan rescinded? An employee received one stating that the purpose of the plan was to improve deficiencies in performance since January 2017. The plan was not indicative of the work or representative of an entire performance year. The issue is that the same employee received a merit increase in October 2016 and another in February 2017 for performing above expectations. The employee received two bonuses in the same cycle.” – Anonymous, Virginia