Related Entry within SuperTable
You can’t use this special Twig function to get reverse related entries at all, as you’re not dealing with reverse relations in that context.

From a stageEvent entry you can just loop through the Super Table rows and get the related artist entry like so:

{% for row in entry.lineUp %}
    {% set artistPerforming = row.artistPerforming.first() %}

    {% if artistPerforming %}
        {{ artistPerforming.title }}
    {% endif %}
{% endfor %}