While the database was upgraded to use a 2000 character invoice_note field and the invoices.php was updated to 2000 characters accordingly, it seems that the settings.php file was overlooked and is still set to only 255 characters. If you try to edit your note and then save it, you will get an error that states only 255 characters are permitted in this field.
Simply change settings.php from 255 to 2000 (Search for 255, it’s the only 255 in the file and replace it with 2000).