Purchase order should be in approved state since the auto approve setting is set to 'Y' at store level.
As per my understanding the purchase orders are not linked with store. Please see - http://demo-trunk-ofbiz.apache.org/ordermgr/control/orderentry in the purchase order section there is no store to select. Instead we have the company to select, if needs to do so then we may need to use PartyAcctgPreference entity to store such data.
If community agree then we can try to add that improvement, which may require some discussion before proceed. I would say this is not bug, either it is custom requirement or an improvement.
Store level settings are specific to Sales Order. Also I agree from Rishi that purchase order are not linked with store. We can consider this as the custom requirement and no one has provided patch for this task so we can close it. Also we can't call it Bug.
Changing its resolution type.