To be clear, this appears to be a product flaw or limitation, not necessarily a "bug" that can be fixed via the normal product investigation. Quickbooks Online Support has outright said that this is not an issue that can be "investigated" through the normal channels.
Vendors can send invoices (bills) via Paypal. How they designate item quantities affects whether or not it will import / sync with Quickbooks Online using the Paypal app features.
Any time an invoice (bill) is sent with any item line that has a zero-quantity, the transaction cannot import from Paypal to Quickbooks Online.
The issue is not resolved using the following navigation in Quickbooks Online, Apps (tab) > My Apps > Paypal > Settings > Fix Errors > FAQ/other.
Again, this appears to be the Paypal app working as-designed, and is simply a flaw or limitation of the app's design.
Primarily, the app simply needs to be reconfigured to accommodate or omit zero-values to allow syncing of those transactions. It is not feasible to inform senders to accommodate or update their invoices (bills) to work-around this type of product issue. Please advise or respond if this is an issue that can be addressed.