FCM legacy http throw random (502) Bad Gateway exception

I have an Asp.net that makes about ~100K requests/day on FCM legacy HTTP end point https://fcm.googleapis.com/fcm/send

But every day about 500 randomly requests throw (502) Bad Gateway, and if I tried these requests manually after a while it works and the notification arrives.

Error I get:

Error 502 (Server Error)!!1
<p><b>502.</b>
<ins>That’s an error.</ins>
<p>The server encountered a temporary error and could not complete your request.<p>Please try again in 30 seconds.  
<ins>That’s all we know.</ins>

Requests that sometimes throw exceptions:

{
   "registration_ids":[
      "token 1",
      "token 2"
   ],
   "data":{
      "type":"navigation",
      "routeName":"Order",
      "Id":"18360"
   },
   "notification":{
      "title":"New Order Placed Successfully",
      "body":"Click here for more details #8073",
      "badge":1,
      "sound":"default"
   }
}

Best regards