Error when submitting block on testnet:{"result":"inconclusive","error":null,"id":"1"} #13027

I am testing bitcoin mining on bitcoin testnet, and got this errors when submitting block: {"result":"inconclusive","error":null,"id":"1"}

I am running bitcoin testnet node on a ubuntu(16.04 x64) with official bitcoin core binary:

  "version": 160000,
  "subversion": "/Satoshi:0.16.0/",
  "protocolversion": 70015,
  "localservices": "000000000000040d",
  "localrelay": true,
  "timeoffset": 0,
  "networkactive": true,
  "connections": 8,
  "networks": [
    {
      "name": "ipv4",
      "limited": false,
      "reachable": true,
      "proxy": "",
      "proxy_randomize_credentials": false
    }, 
    {
      "name": "ipv6",
      "limited": false,
      "reachable": true,
      "proxy": "",
      "proxy_randomize_credentials": false
    }, 
    {
      "name": "onion",
      "limited": true,
      "reachable": false,
      "proxy": "",
      "proxy_randomize_credentials": false
    }
  ],
  "relayfee": 0.00001000,
  "incrementalfee": 0.00001000,
  "localaddresses": [
  ],
  "warnings": "Warning: unknown new rules activated (versionbit 28)"
}

{
  "chain": "test",
  "blocks": 1293495,
  "headers": 1293495,
  "bestblockhash": "00000000000002ef4d1c56e766a3e5d6a4f228686b55fc7b21bc1c4cbbfece4d",
  "difficulty": 5566188.854205163,
  "mediantime": 1524130154,
  "verificationprogress": 0.9999987075717782,
  "initialblockdownload": false,
  "chainwork": "00000000000000000000000000000000000000000000003b4306cf7074524c1f",
  "size_on_disk": 13114557750,
  "pruned": false,
  "softforks": [
    {
      "id": "bip34",
      "version": 2,
      "reject": {
        "status": true
      }
    }, 
    {
      "id": "bip66",
      "version": 3,
      "reject": {
        "status": true
      }
    }, 
    {
      "id": "bip65",
      "version": 4,
      "reject": {
        "status": true
      }
    }
  ],
  "bip9_softforks": {
    "csv": {
      "status": "active",
      "startTime": 1456790400,
      "timeout": 1493596800,
      "since": 770112
    },
    "segwit": {
      "status": "active",
      "startTime": 1462060800,
      "timeout": 1493596800,
      "since": 834624
    }
  },
  "warnings": "Warning: unknown new rules activated (versionbit 28)"
}

Is it a known issue? I met this issue for several times. Is there a way to get rid of this?

Thanks very much!

I've figured out the reason, which is when the testnet diff is set to 1, it becomes a bit competitive for block submitting.