Tax deducted then added when renewing membership

To reproduce:

• setup invoicing and taxes. create the financial account and assign to the member fee type
• create a new membership
• check the contribution and confirm the tax was applied properly
• renew the membership as an admin. note that on the renewal form the amount due does not reflect the tax (when creating the membership it does)
• review the contribution. no tax has been applied. if you generate an invoice, the unit price appears as the amount less tax, and then the tax is later added – so the resulting total due is the base amount

I suspect the oddity with the invoice is because it recognizes the line item as a taxable value, and so subtracts the taxable amount from the unit price in order to show how it's added later. in other words – I think the problem is with the initial transaction (renewal) – the tax is not being calculated there.