Unable to get local buffer in class protected method in chain of command in X++
useExistingReportData() returns true, and therefore the data creation is skipped, if the data was already created for the same transaction. I suspect it's not really the case (that you failed to stop in those methods in debugger may mean that you didn't configure debugging correctly), but even if it is, you can't use that, because you want to intitialize the data and not using old data without your fields being initialized.
First of all, put a breakpoint to SalesInvoiceDPBase.processReport() and check what happens there. If your breakpoint is ignored, enable debugging of Application Suite model (Dynamics 365 > Options > Debugging).