When Sort use _ignoring_unmapped_fields how search_after do?

In ES version6.3.2
I am confusing is it a bug or a feature as following descriptions:

  1. when use sort _ignoring_unmapped_fields,
    why there is ambiguous response in unmapped_type
    WHEN USE type:float/double
    PUT test_index/_search { "sort": { "unknown_field": { "unmapped_type": "float/double" }, "_id": { "order": "asc" } } }
    RESPNSE in sort
    "sort": [ "Infinity", "xxxID" ]
    WHEN USE type:byte/boolean/integer
    PUT test_index/_search { "sort": { "unknown_field": { "unmapped_type": "byte/boolean/integer" }, "_id": { "order": "asc" } } }
    RESPNSE in sort is INT64MAX
    "sort": [ 9223372036854775807, "xxxID" ]
    WHEN USE type:string/keyword
    PUT test_index/_search { "sort": { "unknown_field": { "unmapped_type": "string/keyword" }, "_id": { "order": "asc" } } }
    RESPNSE in sort is null
    "sort": [ null, "xxxID" ]
  2. when use unmapped_fields how a search_after do to ignore these value ?

so i issued: https://github.com/elastic/elasticsearch/issues/44904