Bug: Can't set assignee after removing initial assignee

**Summary**

Reported by a user in the community: After creating and assigning a user, when the initial assignee is removed they can’t set another assignee other than themself (see screen recording attached)

**Steps to reproduce:**

* create task and assign to user A
* in the task details remove user A as the `assignee`
* try to add a new user to `assignee` but not able to
* the only user I can assign to this task is now myself

**Actual Result**

User can only add themself as an assignee after issue has been created.

**Expected Result**

User should be able to add any project member as the assignee for the issue.

_{Huly®: UBERF-8522}