Same issue. Here's the problem: Your system appears to be too smart!

The receipts I captured with the mobile app disappeared off the app without warning, and when I went to the web app, they weren't in the usual place, under "Available receipts". Because I (and all the rest of your users) are used to finding my receipts under "Available Receipts", and assigning them manually, it did not occur to me that your system would have automatically assigned them to the right expense! Your UX people should take this into account - sudden changes without sufficient prompts will make the system less usable, not more.

Best,

Gabe