Relevant telegraf.conf:

[[inputs.file]]
files = ["data.json"]
data_format = "json"
tag_keys = [
  "Serial",
  "Model",
  "Firmware",
]

System info:

Linux CentOS 7.7
Telegraf v1.12.6

Steps to reproduce:

Input Data:

{
  "device0": {
    "Count": 0,
    "Errors": 0,
    "Serial": "9JHNGTUT",
    "Model": "WDC WUH721414ALE604",
    "Firmware": "LDGSW07G"
  },
  "device1": {
    "Count": 0,
    "Errors": 0,
    "Serial": "9JHLPW9T",
    "Model": "WDC WUH721414ALE604",
    "Firmware": "LDGSW07G"
  }
}

Expected behavior:

file,Firmware=LDGSW07G,Model=WDC\ WUH721414ALE604,Serial=9JHNGTUT device0_Count=0,device0_Errors=0 1578088300000000000
file,Firmware=LDGSW07G,Model=WDC\ WUH721414ALE604,Serial=9JHLPW9T device1_Errors=0,device1_Count=0 1578088300000000000

Actual behavior:

file device0_Count=0,device1_Count=0,device0_Errors=0,device1_Errors=0 1578088240000000000

Additional info:

In opposite, data below will pickup tags but there is no place for path which can be important key

[
  {
    "Count": 0,
    "Errors": 0,
    "Serial": "9JHNGTUT",
    "Model": "WDC WUH721414ALE604",
    "Firmware": "LDGSW07G"
  },
  {
    "Count": 0,
    "Errors": 0,
    "Serial": "9JHLPW9T",
    "Model": "WDC WUH721414ALE604",
    "Firmware": "LDGSW07G"
  }
]

file,Firmware=LDGSW07G,Model=WDC\ WUH721414ALE604,Serial=9JHLPW9T Errors=0,Count=0 1578088310000000000
file,Firmware=LDGSW07G,Model=WDC\ WUH721414ALE604,Serial=9JHNGTUT Count=0,Errors=0 1578088310000000000