@AJD, in what order did you construct this test case? Did you enter the debit note before defining the Depreciation Calculation Worksheet? Or did you define the worksheet and then go back to enter the debit note? Because the worksheet is not a financial transaction, it is very sensitive to the order in which things are done.
The debit note was entered before the worksheet was defined.
In my business file the asset has more invoices and debit notes than I put in the test. The worksheet does not include any of the debit notes.
To get around the issue I have created a BS Account for Work in Progress (WIP) where I have put all the invoices and debit notes. Then on commission date I have journalled the net total across to the Fixed asset account.