This is a very minor bug, but something that should be fixed. I have been able to repeat it a couple times following these steps:
1. You must have a client with 2 or more projects with uninvoiced time
2. Create an invoice from the front dashboard by clicking on the Create invoice link on the right hand side of the project.
3. Note the original invoice amount, then click edit and add the time from an additional project by clicking the checkmark next to the project and clicking save.
4. The new amount is updated in the invoice. Click on edit and the amount in the upper right corner changes to the original amount. The amount at the bottom of the invoice remains correct.

Thanks for the report Ross. I’ll add it to my list and look into it first chance I get.