I installed Amavis and it is working, but bounces are simply vanishing.

Check that you haven't inadvertently set up the transport like this:

amavis:
  driver = pipe
  command = "/usr/sbin/amavis -f ${sender_address} -d ${pipe_addresses}"

The last line should be:

command = /usr/sbin/amavis -f <$sender_address> -d $pipe_addresses

The important thing is the <> around the sender address; removal of the unnecessary "" and {} is just tidying. See the amavis FAQ at http://www.amavis.org/amavis-faq.php3.