I do not know at which version this problem appeared, but it had to have been since July 20, 2016. My version is 16.7.80, so it was before that.
When using Spend Money and allocating to Billable expenses account, the Customer field appears, but no customers are in the dropdown list. Nor can one enter a customer name. The transaction can be saved, and the correct things happen in the relevant cash account, the proper increase occurs in the Billable expenses account on the balance sheet, but obviously nothing appears as Uninvoiced in Customers tab. So it is not possible to create an invoice with the billable expenses. I assume this is a small bug and relatively easy to fix, @lubos.