It is good to keep in mind that actually there is no lesson field in this form. The lesson is only printed to distinguish between assignments when there is more than one item of the same type. (If you were handwriting them, you could use something else to make the distinction)
I wonder if the lesson # appears in the main screen of TB?! It looks like the lesson numbers are not imported at all. If not:
Unzip attachment. In TB | Settings go to the >_ sign in the bottom RH corner. Navigate to the unzipped txt file and execute. Reimport the workbook.