projects api call work on zoho creator but doesn't work when called from other zoho app

Greetings

i have the habit of creating and testing custom functions on zoho creator before moving them to the apps where i want to use them in automated workflows.

i've been testing the zoho projects api call "get users" to get a list of all projects users. this is not included in the default projects connection.

usersList = invokeurl
[
url :"https://projectsapi.zoho.com/restapi/portal/"portal"/users/"
type :GET
connection:"test_projects_connection"
];

i've tried this invokation from an admin and a user account on creator and it worked fine on both.

then i moved my custom function to zoho books where i'm going to use it, i just had to change the connection. the invocation worked only on the admin account. while on the employee account it returned ""code":6401,"message":"Unauthorized Access"}". although it worked fine on the employees creator side. why am i getting this? how can i fix it?