The specs say: the FIRST timestamp is automatically updated. It shows any change in the fields, like mtime. So you may consider adding a second timestamp to the field, which contains only explicitly updated stamps.

The other thing what I have found to work is by adding logged to the update:

   UPDATE tasks
   SET quota = quote -1,  logged = logged
   WHERE member_id = ?
[download]

It's a bit of a hack...