Provisioner local-exec: 'always_run' trigger doesn't work as expected

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In my terraform I have mysql module as follows:

# create ssh tunnel to RDS instance
resource "null_resource" "ssh_tunnel" {
  provisioner "local-exec" {
    command = "ssh -i ${var.private_key} -L 3306:${var.rds_endpoint} -fN ec2-user@${var.bastion_ip} -v >./stdout.log 2>./stderr.log"
  }
  triggers = {
    always_run = timestamp()
  }
}

# create database
resource "mysql_database" "rds" {
  name = var.db_name
  depends_on = [null_resource.ssh_tunnel]
}

When I'm adding new module and running terraform apply first time it works as expected. But when terraform apply running without any changes I'm getting an error:

Could not connect to server: dial tcp 127.0.0.1:3306: connect: connection refused

If I understand correctly, provisioner "local-exec" should execute script every time due to the trigger settings. Could you explain how should it works properly?

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