Same issue is happening for us.  Never used to occur. 

The open PO's in question don't even show up as an open PO in the Vendor Center (when Showing Purchase Orders --> Filter By Open Purchase Orders).

Navigate to the PO, mark it as closed (using the Mark As Closed function) and then mark it as open (using the Mark As Open function) ... and then it shows up in the Vendor Center list as an open PO.

Clearly there is an issue here that QB needs to address!