I have a workflow with two steps, Open and In Progress.
For these two steps, I have added the jira.permission.assignable.group property to jira-developers.

It works fine in the sense that when I try to assign tickets to User_A who is not in the jira-developers group, the user picker doesn't suggest that username.

But I can still assign tickets to User_A (not in the jira-developers group) by typing his username directly into the assignee field. (despite no auto-complete or suggestions)

I believe the jira.permission.assignable.group should override the project's permissions scheme ? User_A DOES have the assignable user permission, but the property configured should override the project's permission scheme and only allow jira-developers to be assigned in the Open and InProgress steps.