Jira REST API for project permissions returns wrong response for "Next Gen" projects

[Georgi Dobrev]

Hello, 

I'm trying to get the project permissions for a portal user with no access to given "next gen" project via REST API and expecting a response with "havePermission":false, but the endpoint returns "true". 
Is there any workaround to resolve the problem - how to get the correct project permissions for "next gen" portal users with REST call?

I'm using this endpoint:

"/rest/api/2/mypermissions?permissions=CREATE_ISSUES&projectId={projectId}"