How to resolve GCP API gateway error 500?

Question

My request flow is :

1.HTTP python client (a script) => 2. GCP API gateway => 3. validate request against firebase => 4. if request valid call cloud function v2 (cloud run)

1. Python script is generating ID token from refresh token by using 'https://securetoken.googleapis.com/v1/token. And doing a request to API gateway using it.

2. API gateway config is al follow.

swagger: "2.0"
info:
  title: Test
  description: API to read validated token details
  version: 1.0.0
paths:
  /test:
    get:
      x-google-backend:
        address: URL TO cloud function
      responses:
        "200":
          schema:
            $ref: '#/definitions/UserDetails'
          description: Successful response
      description: Returns details.
      operationId: testID
      summary: Get details from xyz
      security:
        - firebase: []
  .
  .
  .

definitions:
  UserDetails:
    title: Root Type for UserDetails
    description: User details object created from information in headers
    type: object
    properties:
      userId:
        type: string
      email:
        type: string      
securityDefinitions:
  firebase:
    flow: implicit
    authorizationUrl: ''
    scopes: {}
    type: oauth2
    x-google-issuer: "https://securetoken.google.com/*********"
    x-google-jwks_uri: "https://www.googleapis.com/service_accounts/v1/metadata/x509/securetoken@system.gserviceaccount.com"
    x-google-audiences: "******"

1. This flow works fine but in between API gateway throws an error 500: Here are logs from gateway

{
  "insertId": "8310ad5-bf4-43b-966-d94485090890563961239@a1",
  "jsonPayload": {
    "http_status_code": 500,
    "location": "us-central1",
    "api_version": "1.0.0",
    "api_name": "removed",
    "producer_project_id": "removed",
    "service_agent": "ESPv2/2.35.0",
    "log_message": "removed",
    "response_code_detail": "via_upstream",
    "timestamp": 1664181080.6733284,
    "api_key_state": "NOT CHECKED",
    "api_method": "removed"
  },
  "httpRequest": {
    "requestMethod": "POST",
    "requestUrl": "/test",
    "requestSize": "5411",
    "status": 500,
    "responseSize": "599",
    "remoteIp": "600:1900:200:a3::16",
    "latency": "0.009s",
    "protocol": "http"
  },
  "resource": {
    "type": "api",
    "labels": {
      "method": "removed",
      "version": "1.0.0",
      "service": "removed",
      "project_id": "removed",
      "location": "us-central1"
    }
  },
  "timestamp": "2022-09-26T08:31:20.673328479Z",
  "severity": "ERROR",
  "logName": "removed"
}

Note : The client script is scheduled from different places parallely.

Anyone knows root cause ? issue is coming randomly, like sometime for 24 hours it wont come, suddenly it comes, and comes after few minutes again...