As a business user, I entered invoices and payments then I started assigning payments to invoices "Edit Payment"/"Assign as payment" in the right click menu.
In the "Process Payment">Documents subwindow, invoices sorted by date do not appear in order of invoice number in a given day. I entered invoices in the order of invoice number. However, in the "Due Invoices Reminder", sorted by due date, they appear as sorted by invoice number.
This seemingly random order for invoices posted on the same day made me confused and caused to think that assigning a payment to multiple invoices was not working.