searching on projectDFF fields under distributions using the purchaseOrders endpoint

Summary:

We need to search using the query parameter (q) on the field "lines.schedules.distributions.projectDFF._PROJECT_ID" using the purchaseOrders endpoint, but are getting an error.

Content (required):

We are trying to serach using the above criteria but get an error Failed to build ViewCriteria from expression \"lines.schedules.distributions.projectDFF._PROJECT_ID=1058\" using view object PurchaseOrder1. Unable to create criteria item from node: \"lines.schedules.distributions.projectDFF._PROJECT_ID\"

If we try to search on another field at this level withiut an underscore at the start it is successful e.g. lines.schedules.distributions.projectDFF.PoDistributionId

Not sure if this is a syntax error due to the underscores that we can escape, or if these fields are not available for use with the query parameter.