We have found a problem with some tests. The random questions which are stored on the table mdl_question are disappearing, as well as the answers of the students who had responded, which are stored in mdl_question_states, and the grades of the tests (mdl_quiz_attempts).
This is affecting to a lot of tests.

We are using a version 1.6.4 with PostgreSQL.

Luckily, we have backups of the database, but we would like to find out why this is happening. We have thought that it could be the VACUUM ANALIZE done by PostgreSQL.

Cheers