Solution quotation number is getting populated in Sales Order Header Customer Reference field

When a sales order is being created as a follow up document from Solution quotation, Solution quotation number is getting updated in Customer Reference field in Sales Order Header, which is used to capture Customer Purchase Order number on SD side and we are loosing track of Customer PO number at Sales Order header level.
I am passing External Reference number in Solution quotation, but it is not getting updated at Sales Order Header - Customer Reference field (VBAK -BSTNK) and rather it gets updated as Solution quotation number.

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