For the Purchase Order # (PO#) to show up on a bill in Bill.com, from a bill that was created out of a PO and synced over from Sage Intacct, the PO# must be in the "Reference Number" field of the bill in Sage Intacct.
Sage Intacct does not auto populate the Reference Number field with the PO#. That will have to be entered manually.
For those that are familiar with custom scripts, please consult with Sage Intacct regarding auto populating the reference number field with the PO# via a custom script.