You cannot sort and/or filter by calculated fields. If you want to filter/sort, you're going to need to create a FlowField on the Customer Ledger Entry table and use that on the page. Then you'll be able to sort/filter.
Your code to set "Bill-to Customer No." is only inside OnInsert trigger. When you "change your mind" later, then you don't do an insert anymore, but do a Modify.
My suggestion would be to move that code to the OnValidate trigger of "Job No." field.
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