It was a mistake; I'm new to editing OpenStreetMap tags.

Using the StreetComplete app, I marked this as a "roof" building type, since I couldn't find anything that fit better.

Then, later, using the default editor, I changed the "Feature Type" from "Roof" to "Amphitheater," since I was pleased to find there was a feature type that fit better.

I didn't realize that the "roof" and "amphitheater" tags could exist together. Thank you for re-adding the "roof" tag! :)