In the development environment every statement is logged, however for some reason the DEALLOCATE's never appear in the logs. This could be caused by the view not recording the prepared statement, as the DEALLOCATE is only made if the prepared statement name exists in the view.
max_prepared_transactions = 6 is the value set in postgresql.conf.
I don't know why that particular record exists but your comment about the application not creating it and DEALLOCATE's never appearing in the log suggests to me a misunderstanding of what this view shows.