[N1QL+FTS] Invalid mapping error with scope.collection.docid_prefix filter using "_" as delimiter

Executing the following query results in error :

SELECT RAW meta().id

FROM `09762c`.`_default`.`09762c_2` AS t

WHERE SEARCH(t, "09762c_1", {"index": "repro"});

Error : 

[

  {

    "code": 5010,

    "msg": "Error evaluating filter",

    "reason": {

      "caller": "util:336",

      "code": 5000,

      "key": "Internal Error",

      "message": "n1fty: index mapping not found"

    }

  }

]

The index definition :
{
"name": "repro",
"type": "fulltext-index",
"params": {
"doc_config":

,
"mapping": {
"default_analyzer": "standard",
"default_datetime_parser": "dateTimeOptional",
"default_field": "_all",
"default_mapping":

,
"default_type": "_default",
"docvalues_dynamic": false,
"index_dynamic": true,
"store_dynamic": false,
"type_field": "_type",
"types": {
"_default.09762c_2": {
"dynamic": false,
"enabled": true,
"properties": {
"testUid": {
"enabled": true,
"dynamic": false,
"fields": [

]
}
}
}
}
},
"store": {
"indexType": "scorch",
"segmentVersion": 15
}
},
"sourceType": "gocbcore",
"sourceName": "09762c",
"sourceUUID": "ac6fc7e3c19f828650f38fa9c7cec65a",
"sourceParams": {},
"planParams": {
"maxPartitionsPerPIndex": 1024,
"indexPartitions": 1,
"numReplicas": 0
},
"uuid": "25a7e9bbef1b4897"
}
If you change the prefix from __ to ::, everything works as expected.

Here is a sample of documents so you can test this :

[

  {

    "id": "09762c_1::0",

    "testUid": "09762c_1"

  },

  {

    "id": "09762c_1::1",

    "testUid": "09762c_1"

  },

  {

    "id": "09762c_1::2",

    "testUid": "09762c_1"

  },

  {

    "id": "09762c_1::3",

    "testUid": "09762c_1"

  },

  {

    "id": "09762c_1::4",

    "testUid": "09762c_1"

  },

  {

    "id": "09762c_1::5",

    "testUid": "09762c_1"

  },

  {

    "id": "09762c_1::6",

    "testUid": "09762c_1"

  },

  {

    "id": "09762c_1::7",

    "testUid": "09762c_1"

  },

  {

    "id": "09762c_1::8",

    "testUid": "09762c_1"

  },

  {

    "id": "09762c_1_0",

    "testUid": "09762c_1"

  },

  {

    "id": "09762c_1__0",

    "testUid": "09762c_1"

  }

]