Scenario
If Tax Reporting is on Cash Basis, only realized transactions will be displayed in the Business Activity Statement and GST on Sales Report.
Solution
To verify this, you can navigate to Setup > Accounting > Set Up Taxes > Tax Reporting Cash Basis checkbox. If the field is marked, Tax Reporting is indeed on Cash Basis. If unmarked, transactions that are yet to be realized will be included in the Business Activity Statement and GST on Sales Report (including standalone credit memos).
You can also follow the steps below to temporarily view the standalone credit memos in the GST on Sales Report:
1. Navigate to Reports > VAT/GST > GST on Sales Report
2. Click on the Customize button
3. Go to More Options tab
4. Set Cash Basis = False
5. Rename the Report then click Save
Note that this is only for report viewing purposes and will not affect the figures in the Business Activity Statement.