Unable to select assignee for a subtask from parent issue

Issue Summary

From a parent ticket (in the new issue view), an assignee cannot be selected in-line for a subtask, if the subtask does not already have an assignee. In order to select an assignee, the subtask must be opened. After an assignee is selected, then this can be changed inline from the parent ticket.  

Note that this behaviour is also inconsistent with issues from an epic - where assignees can be selected in-line for issues with no assignees, from the epic ticket. 

Steps to Reproduce

1. Create an issue (parent ticket)
2. Create a subtask with no assignee
3. From the parent ticket, try to select an assignee in-line (without clicking into the subtask)

Expected Results

From the parent ticket, I should be able to select an assignee through an inline action (without having to open up the subtask). Similar to selecting assignees for issues within an epic. 

Actual Results

There is no way to select an assignee for a subtask, from the parent ticket, without opening up the subtask. 

Workaround

Set an Automation rule whereby a specified user is assigned when a subtask is created