Bulk Change assignee modifies selected issues assignee to unassigned

Issue Summary

Performing the bulk change assignee is not reflected. The assignee chosen in #assignee-field lost/Unassigned when you select an assignee other than the issue reporter.

This is reproducible on Data Center: Yes

Steps to Reproduce

1. Filter issues that need to be bulk change assignee.
2. Click on tools bulk change
3. Choose Issues and click Next
4. Click on Edit issue 
5. Change Assignee to some value other than issue reporter and click next then confirm
6. Check the assignee is set to Unassigned   

Expected Results

The assignee value selected in step 5 should be reflected in step 6.

Actual Results

There is no messages printed in logs.

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Workaround

Currently, there is no known workaround for this behaviour. A workaround will be added here when available