Advanced Roadmaps plan do not respect project permissions while an issue is assigned from a plan

Issue Summary

When we click on the assignee field for an issue in an Advanced Roadmap plan, the drop-down will show and allow us to select all the users (including the ones without Assinable user permission or even non-license user). After you select A non-license user the workflow doesn’t allow you to save the changes.

Steps to Reproduce

1. Create a plan with some project issues in it.
2. Pick any issue and click on the assignee column for it.
3. You will notice that those users who lack Assignable user permission will also show up in the list and the plan allows you to select it.
4. When you go to save the changes, the workflow prevents this final step and even though the changes are saved, the user is removed from the assignee field.
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The issue is reproducible across different instances.

Expected Results

Only users with assignable user permissions should show up in the list and we should be able to only select those users. 

Actual Results

All users are showing up (including non-license users)

Workaround

No workaround is available.