Here is a recreation:

DELETE index

PUT index
{
  "mappings": {
    "doc": {
      "properties": {
        "my_date": {
          "type": "date", 
          "format": "yyyy/MM/dd"
        }
      }
    }
  }
}

PUT index/_mapping/doc
{
  "doc": {
    "properties": {
      "my_date": {
        "type": "date", 
        "format": "yyyy-MM-dd"
      }
    }
  }
}

GET index/_mapping

date allows its format to be updated. This is trappy because then it could be changed to a format that doesn't work with already indexed documents, or even worse: a format that parses to a different date.

I think we need to ensure that the format either cannot be changed, or make it a list that is append-only so that it is only possible to append date formats.