Most 2D maps and some 3D maps use only the shapes with a tag building=yes. That’s the outline. It should cover the whole area under the building.
More advanced renderers use the shapes with building:part=yes. That’s the 3D shapes. Here you have just one. You probably want another one for the base of the tower.
Editing this is tricky; current outline is not an actual outline since it covers only the base, but making another shape for outline using the nodes of the upper part, moving just the tags, and using the old shape for building:part probably hurts change history.
Looking at your relation, it seems you only mapped one building part. Going by the book, there should be three members of that relation:
the lower part
the upper part
the outline for the building as a whole (which e.g. should have the total height of the building).
Also, Simple 3D Buildings asks that the building outline should surround all the parts (rather than just the ground level footprint), but for a renderer that supports building relations, this should not make a difference.
Both 3D-maps don’t fully support Simple 3D Buildings.
F4map seems not to support relations and ignores building:parts overlapping the outline. See “Buildings & building:part” on https://wiki.f4map.com/render.
If you tag the outline with both, building=yes and building:part=yes together, OSMB should show it. But that’s not the way it’s meant to be by Simple 3D Buildings (as far as I understand it).
F4map won’t show the upper part of the tower unless the outline of the building is at least as large as the parts. Thats’s the case with the example at Singapore Airport, JaLooNz hinted to.
What is the favoured outline? The footprint on the ground or the largest extent?
Simple 3D Buildings explicitly asks for the largest extent. I don’t think there’s a clear preference otherwise, although aerial imagery makes it easier to identify the largest extent than the footprint on the ground.