Task rules Slack notification appear wrong

[Claudio Mastrapasqua]

Hello,
can you help me? Why following task_high_memory_usage_1g defined task rule (default):

  - alert: task_high_memory_usage_1g
    expr: |
      sum(container_memory_rss{container_label_com_docker_swarm_task_name=~".+"})
      BY (container_label_com_docker_swarm_task_name, container_label_com_docker_swarm_node_id) > 1e+09
    for: 5m
    annotations:
      description: '{{ $labels.container_label_com_docker_swarm_task_name }} on ''{{ $labels.container_label_com_docker_swarm_node_id }}'' memory usage is {{ humanize $value }}.'
      summary: Memory alert for Swarm task '{{ $labels.container_label_com_docker_swarm_task_name }}' on '{{ $labels.container_label_com_docker_swarm_node_id }}'

Appers in Slack like below?

No description or other annotations.

task_high_cpu_usage_50 task rule appears correctly:

Thank you.