Weird type of importing array field in Solr document

I'm using solr json api to import documents into solr. one field has an array of integers, when I import the values into document some documents have weird structure to store values.

The way most documents are imported (desired):

      {
        "id":"2a0424d1-5b53-4014-8450-c996cb60dfd1",
        "searchable_categories":[210,
          205,
          328,
          329,
          262,
          1086,
          1090,
          1305],
          ...
      }

but some documents are imported like this:

       {
        "id":"7f6a72e6-bcdb-4f87-a038-78e65323d557",
        "searchable_categories.0":[210],
        "searchable_categories.1":[2421],
        "searchable_categories.2":[217],
        "searchable_categories.3":[771],
        "searchable_categories.4":[262],
        "searchable_categories.5":[796],
        "searchable_categories.6":[1343],
        "searchable_categories.7":[1349],
        "searchable_categories.8":[1413],
        "searchable_categories.12":[1597]
        ...
        },

The value format is the same when populating json data. Does anybody know what's going on?