Hi Yunkin ,
Apologies for any inconvenience caused.
PO Pro uses the default 'trash' post-status to store the Archived PO orders. We never delete them (indeed, there's no action to do so in the PO Pro pages).
Did you execute a cleaning tool or something similar to clean your posts? Maybe that could affect the archived POs...
The inventory levels won't be changed when a PO is deleted, so you shouldn't worry about that.
Best Regards,
José Andrés