I think it is a bug, but it was rejected by Microsoft to fix :(((
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Problem: Invoice description is blank when invoice is posted via Invoice pool

1. Create a purchase order
2. Register an invoice
Note: you can enter a text in the Description field in the register, but it won’t flow into the invoice description too.
3. Go to Invoice pool and having the invoice selected click Purchase order to match it
Post the invoice and check the results by going to the purchase order invoice
Issue: Invoice description field is blank on the posted invoice