why a purchase order is approved automatically when it comes from an approved requisition?

This happens only with purchase orders created from a requisicion in iProcurement.

For example I defined the rule with the attributes, ORG_ID of the Operating Unit, ITEM_CATEGORY, and Monto is Greater than 0

and three Approval groups.

In the Test_Workbench it appears as you are taking the rule, but automatically approves.

Regards