Sending POST messages using endpoint "me/messages" : code: -1, error_code: 2018012

Question

We are requesting the pages_messaging permission, we have submitted request multiple time and it's goes rejected by saying Bot is not responding. During testing with a test user the functionality performed seamlessly using the "me/messages" endpoint. However, we encountered an issue when investigating why the app reviewer wasn't receiving replies. Specifically, we identified the error as "ERROR CODE: 2018012" when testing from the Graph API Explorer.

In last request when web-hook App reviewer (recipient id : XXXXX93418XXXXXX) By using this Id we get an error response Below:

{
  "error": {
    "message": "(#-1) Unexpected internal error",
    "type": "OAuthException",
    "code": -1,
    "error_subcode": 2018012,
    "fbtrace_id": "ACzSlcFJtPOv3hATV2NmOVV"
  }
}

When we test with the Test User (recipient id : XXXXX47110XXXXXX) we got a successful response below

{
  "recipient_id": "XXXXX47110XXXXXX",
  "message_id": "m_G1HHOz1BbrO5bVV6..."
}

We are trying to get the pages_messaging permission for our Facebook app messenger we have implemented a bot for automate send response from back-end when we will receive the web-hook, Expecting while Facebook are reviewing our permission request they should get the successful response mention in description, but somehow they are not able to get a response from bot.