I know we don't guarantee backward compatibility of backups but it looks like quite a big regression.
It was discovered while testing MDL-44630 and jmvedrine mentioned that it is a regression from MDL-43670 (I did not check).
Attached is a backup file of a course (made in 2.7) that contains a quiz with 4 questions. When I restore this backup in 2.6 the questions disappear. Also there were plenty of notices during restore under the progressbar but they disappeared faster than I could copy them