Hi Brian E,
As there is less ability to modify the legend in a web scene as opposed to a web map, you will need to toggle this off from your hosted feature layer's item details page. Select Visualization > then Change style > choose Options under Types (unique symbols):
Then toggle off the checkbox for 'Other' (shown above) > select OK > then Done > then Save the layer. Then when adding the layer to a web scene, 'Other' will be absent from the legend.
Hope this helps, let me know if there are any questions about this workflow.
Thanks for the response. My mistake, I should have assumed you were working with a scene layer. You're right, this is a bug - I found the record internally. I logged it as BUG-000132826 so we can better track progress on it, and know when other customers run into it. If you would like to attach your account please email me at firstname.lastname@example.org with your customer number.