Why does the assignee added to workflow using the 'Add Assignee' option disappear after being added?

Summary:

The 'Add Assignee' workflow option is not working as expected. Within the workflow user 'Actions' dropdown, after clicking 'Add Assignee' and adding the necessary user to the workflow, the user disappears from the existing workflow. I acknowledge an assignee can only be added to an existing workflow that has more than one approval level; however, I am unable to add an assignee in instances where there are more than one levels of approval.

Has a solution been found for this issue?

Thanks

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