No handler for type [string] declared on field [name]

Hi community.

We have one issue after rolling upgrade from version 5.x to version 6.x.
The issue is happening when reindex. Please check this out below:

     {
       "took": 22,
       "timed_out": false,
       "total": 205,
       "updated": 0,
       "created": 0,
       "deleted": 0,
       "batches": 1,
       "version_conflicts": 0,
       "noops": 0,
       "retries": {
         "bulk": 0,
         "search": 0
       },
       "throttled_millis": 0,
       "requests_per_second": -1,
       "throttled_until_millis": 0,
       "failures": [
         {
           "index": "logstash-dev-2020.09",
           "type": "logevent",
           "id": "AXROtw0f8TodnXvQuCn4",
           "cause": {
             "type": "mapper_parsing_exception",
             "reason": "Failed to parse mapping [_default_]: No handler for type [string] declared on field [@version]",
             "caused_by": {
               "type": "mapper_parsing_exception",
               "reason": "No handler for type [string] declared on field [@version]"
             }
           },
           "status": 400
         },
     .
     .
     .
         {
           "index": "logstash-dev-2020.09",
           "type": "logevent",
           "id": "AXRzHrS2AW9Oz8lG34J2",
           "cause": {
             "type": "mapper_parsing_exception",
             "reason": "Failed to parse mapping [_default_]: No handler for type [string] declared on field [@version]",
             "caused_by": {
               "type": "mapper_parsing_exception",
               "reason": "No handler for type [string] declared on field [@version]"
             }
           },
           "status": 400
         }
       ]
     } 

Is there anyway to solve above issue?
Thanks in advance.