Hi Arpit,
This is Standard SAP Behavior.
You can Control this by little customization in Purchase Order Form.
Create one UDF of Quantity in Form at Row Level and then Create FMS to Copy Standard Quantity Field in that UDF and Put Auto Refresh on Standard Quantity SAP Filed.
Now in GRPO Create One FMS to Copy That UDF Quantity in Stanadard Quantity field and Put Refresh of the FMS on Stock UOM.
Through this way you can control this.
Hope this help
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Regards::::
Atul Chakraborty