If the attribute was added in the post editor screen via WooCommrece drectly, it may not be registered as a custom taxonomy term, but as a custom field. I’m not sure how WooCommerce determines which one to use, but sometimes it becomes a custom field.
Try looking for attribute_pa_test or attribute_pa_ or ‘attribute_’ within the indexed custom fields list under the index table options panel: https://i.imgur.com/MgMUVNX.png
If it shows up, select it, and then save the product again, so the new field is indexed.
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