Symptoms

If you have an issue in one project, then try to move it, and the original assignee is not valid in the new project, then you will be prompted to specify a new assignee.

In previous versions the default value there would be "Automatic", however it seems that in 5.1.5, the value there is "-1".

This affects only a single issue move, bulk issue move seems unaffected.

Workaround

For a single issue, start typing "Automatic" and it will be able to use the auto-assign.

You can also get around this by using a bulk change.