Which IIS config causes the "405 Method not allowed" error for specific calls? - asp.net-web-api

Question

In our current WebAPI calls, only a few specific ones are returning the 405 error message (405 Method not allowed), the rest are working okay. The ones that are failing are only failing in a specific environment, in all the other environments all calls are returning 200 and everything is kosher.
Which IIS setting or system setting can cause this ? I am zooming in on this because this looks more like a systems/config issue.
Request URL : <hostname/GetAllStuff?userId=johndoe&sortByCustom=CreatedDate&sortByOrder=desc
Request Method: GET
Status Code: 405 Method Not Allowed
Remote Address: 172.72.78.33:443
Referrer Policy: no-referrer-when-downgrade
Access-Control-Allow-Credentials: true
Access-Control-Allow-Headers: Content-Type
Access-Control-Allow-Methods: GET,POST,PUT,DELETE,OPTIONS
Access-Control-Allow-Origin: *
Allow: GET,POST,DELETE,PUT
Cache-Control: no-cache
Content-Length: 136
Content-Type: application/json; charset=utf-8
Date: Tue, 06 Nov 2018 17:16:28 GMT
Expires: -1
The Response looks like this. What is strange is that ALL the API calls are 1.0 (api-version: 1.0) yet only a couple of calls throw this error and that too in a specific environment.
{
"Error": {
"Code": "UnsupportedApiVersion",
"Message": "The requested resource with API version '1.0' does not support HTTP method 'GET'."
}
}