Changing salesorder_number via zoho flow

Changing salesorder_number via zoho flow

For some reason updating salesorder_number via zoho flow does not stick. 

Flow is triggered by new sales order filtered by sales channel

update sales order:
PO#: CX${trigger.reference_number}
Salesorder_number: CX${trigger.reference_number}

PO# successfully updated but Salesorder_number does not. Input shows updated salesorder_number  ( CX1018 in below pictures ). But update does not show the updated salesorder_number