Hello robgeb!

From the docs, I'm seeing:
"If the type=building relation is present for a building, all building parts must be listed as the relation members with the role part. The building outline must be listed with the role outline. The building parts can be located in any possible way (inside, outside, intersecting, touching) relative to the building outline in the presence of the type=building relation. If there is no type=building relation, an application should treat all building parts within the area of the building outline as part of that building."
osm.wiki/Simple_3D_buildings#Building_relations

As a result, I would agree that the removed way should remain. However, it appears that the correct thing to do is add it to the relation/2853424 with role "outline".

Thanks for bringing this to my attention.

Best,
Bryant