So, I've been wrestling with this issue for a few months. There is no satisfactory solution for me. But here is the explanation:

If you are using Anglo-Saxon Accounting (pre-19) or perpetual (post-19), on your invoice, and entry for COGS is created, which then is tagged with analytic account and displayed on the dashboard as CoGS. If your purchase orders also are attached to the project (why wouldn't they if they are being made for the project), you get "both" costs displayed and therefore will have overblown costs.

One solution that we found is to use continental accounting (not having anglo-saxon setting on) since we defer expenses anyway into a Work in Progress type account. The recognition is done manually at the end of the month for expenses and income if we hit any milestones. HOWEVER, all stock entries have to be done manually and are not displayed on the dashboard, because there is no PO for those.

The second solution is to use budgets for accurate cost/revenue reporting as it only affected by financial entries. I.e. Invoices and Bills. And the Bill value is substracted after posting the final invoice. Which mean you have to suck it with double cost display, but you can use anglo-saxon(perpetual) and stock without any issues or manual entry.

Finally third option is to exclude POs from a Project completely. That will display things correctly.

I have been told we could use analytic distributions/plans. But I haven't figured out how.