Same here. Not sure why this happening. In our case the .docx had 6 pages and the PDF tried to convert the first six, with bits missing in it and then converted it corrected from Page 8 so its done it eventually but you end up with 13 pages with the first six partially converted if that makes sense.
We've tried to re-produce this by downloading the student's submission and uploading it again but we are not able to re-create the problem !
Anyone got any ideas ?