That data source will be fixed in v2020 PU2.  If you need a program fix sooner, please contact support.

I was able to create a UDF in the SO detail that will have the same value as the UDF in the SO header that I need on the SO invoice detail line.  So I am assuming I can just make that field visible on the line of the invoice now, correct?  But my other question is how can I get the SO# of each order to be available on the line level as well on the invoice when batching?  Do I need to create another UDF? 

I was able to create a UDF in the SO detail that will have the same value as the UDF in the SO header that I need on the SO invoice detail line.  So I am assuming I can just make that field visible on the line of the invoice now, correct?  But my other question is how can I get the SO# of each order to be available on the line level as well on the invoice when batching?  Do I need to create another UDF? 

Yes you can create an invoice detail UDF with data source from a sales order UDF, then add it to the invoice entry grid.

As Kevin said, there is an AppliedSalesOrderNo field in the invoice detail table.  It is hidden from view if you only have 1 sales order on an invoice.