getting Session state protection violation error for a hidden item that is being set with computation before header - Oracle APEX

For your hidden item P613_EMPLOYEE_TYPE_LABEL set the Value Protected attribute to NO.

When Value Protected of an hidden item is set to YES a checksum is generated when your page is loaded. When you submit the page with a different value the checksum is not valid anymore and you get the error.

"Value protected" only shows for hidden items. This error shows also when you update a visible item that is read only. At least in rel 19.2. See this question also: Can't disable session state protection on Oracle APEX 18.1.0.00.45 for Dynamic Actions updating Page Items