Realm function runAsSystemUser doesn't work / How to execute function as system user

I have a function called sys_function in realm which I want to call/execute/run as system user always, so I chose the “Script” Authentication method and put

exports = function(payload) {
  return { "runAsSystemUser": true }

in the “Execute as a user chosen dynamically by calling a function” field (like adviced in this article: where it says

" The function must return a specific user’s id string or can specify a system user by returning { "runAsSystemUser": true } ."

If I then call this function from another function by context.functions.execute(“sys_function”, <args…>), sys_function still runs as the same user that called the other function.

I looked all over the internet and MongoDB Forums and couldn’t find an answer. I need this to work. Thanks in advance!