Your title is misleading. You are not getting any missing values for date (variable named PERIOD_DT) after PROC TRANSPOSE.
You are getting missing values for other variables. Why? Because before 202307, COUNT_LEVEL contains 3 letter values. At 202307, COUNT_LEVEL has some four letter values. So that's why the variables ACC EXN and TRA are missing, they don't exist at 202307.
From now on, we want you to provide data as SAS data step code as explained. We cannot work from data in screen captures. Please acknowledge that you understand this.
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Paige Miller