Painless stored script "dynamic" compile error

[karasick (Oleksii Kharchenko)]

Can someone please explain to me the next error ?

"error": {
          "type": "illegal_argument_exception",
          "reason": "failed to execute script",
          "caused_by": {
            "type": "general_script_exception",
            "reason": "Failed to compile stored script [simple-test-script] using lang [painless]",
            "caused_by": {
              "type": "circuit_breaking_exception",
              "reason": "[script] Too many dynamic script compilations within, max: [1/5m]; please use indexed, or scripts with parameters instead; this limit can be changed by the [script.context.update.max_compilations_rate] setting",
              "bytes_wanted": 0,
              "bytes_limit": 0,
              "durability": "TRANSIENT"
            }
          }
        }

To reproduce it you can easily follow the next steps on Elastic Cloud:

PUT _cluster/settings
{
    "persistent": {
        "script.context.update.max_compilations_rate": "1/5m"
    }
}

PUT /_scripts/simple-test-script
{
  "script": {
    "lang": "painless",
    "source": """
        ctx._source['test_field'] = 'test value'
    """
  }
}

PUT /test-index

POST /test-index/_bulk
{"update":{"_id":"1"}}
{"script":{"id": "simple-test-script"}, "upsert": {}, "scripted_upsert": true}
{"update":{"_id":"2"}}
{"script":{"id": "simple-test-script"}, "upsert": {}, "scripted_upsert": true}

Why does simple-test-script dynamic at this point ? And what does dynamic mean in this situation ?