Query related to process loaded data rule

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Hi,

When I run the rule for a specific employee,job and with correct start month for that employee the rule(process loaded data) works fine,but if I select any wrong start month then all the values are deleted.For example if my start month is mar for an employee and I run the rule with start month as mar then it works fine,but instead of mar if I select jan or Feb it will  delete all my calculated values for that employee(which I have calculated previously).This problem does not occur when I select period after the start month,like Apr in my example.