Container definition deployed by terraform loses mount_points and port_mappings

I am trying to deploy an aws_ecs_task_definition with two containers that share an EBS volume. Terraform will successfully run the deployment and update the task/container definitions but port_mappings and mount_points are never present in the container definitions when view in AWS.

locals {
  volume_name = "ebs_test"
}

resource "aws_ebs_volume" "ebs_test" {
  availability_zone = "zone"
  size = 51

  tags = {
    Name = local.volume_name
  }
}

data "aws_ecr_image" "container_1_image" {
  repository_name = "Container1"
  image_tag = "latest"
}

data "aws_ecr_image" "container_2_image" {
  repository_name = "Container2"
  image_tag = "latest"
}

resource "aws_ecs_task_definition" "test_task" {
  family = "container_family"
  requires_compatibilities = ["EC2"]
  execution_role_arn = aws_iam_role.ecs_task_execution_role.arn
  task_role_arn = aws_iam_role.execution_role.arn
  placement_constraints {
    type = "memberOf"
    expression = "ec2InstanceId == ${module.instance.instance_id}"
  }

  volume {
    name = local.volume_name
    docker_volume_configuration {
      scope = "shared"
      autoprovision = true
      driver = "rexray/ebs"
    }
  }

  container_definitions = jsonencode([
    {
      name = "Container1"
      memory = 256
      essential = true
      image = "${var.account_id}.dkr.ecr.${var.region}.amazonaws.com/${data.aws_ecr_image.container_1_image.repository_name}:latest@${data.aws_ecr_image.container_1_image.image_digest}"
      mount_points = [
        {
          container_path = "/mnt/${local.volume_name}"
          source_volume = local.volume_name
        }
      ]
      port_mappings = [
        {
          host_port = 80
          container_port = 80
        }
      ]
    },
    {
      name = "Container2"
      memory = 256
      image = "${var.account_id}.dkr.ecr.${var.region}.amazonaws.com/${data.aws_ecr_image.container_2_image.repository_name}:latest@${data.aws_ecr_image.container_2_image.image_digest}"
      mount_points = [
        {
          source_volume = local.volume_name
          container_path = "/mnt/${local.volume_name}"
        }
      ]
      port_mappings = [
        {
          host_port = 80
          container_port = 80
        }
      ]
    }
  ])
}

This is what is always present in the task JSON pulled from AWS:

"portMappings": [],
"mountPoints": [],

Terraform apparently isn't seeing the port_mappings or mount_points properties being set when checking terraform plan:

      ~ container_definitions    = jsonencode(
          ~ [
              ~ {
                  - cpu              = 0
                  ~ memory           = 256 -> 512
                  - mountPoints      = []
                    name             = "Container1"
                  - portMappings     = []
                  - systemControls   = []
                  - volumesFrom      = []
                    # (4 unchanged attributes hidden)
                },
              ~ {
                  - cpu              = 0
                  - mountPoints      = []
                    name             = "Container2"
                  - portMappings     = []
                  - systemControls   = []
                  - volumesFrom      = []
                    # (5 unchanged attributes hidden)
                },
            ] # forces replacement
        )

I have been trying to use this GitHub repo is a reference: https://github.com/markgllin/ecs_with_ebs

What am I missing?

You are using the names mount_points and port_mappings in your Terraform code, but Terraform is looking for the names mountPoints and portMappings (as you can see in the Terraform plan output).