Hi, multipolygons are invalid with outers joined this way.

https://tools.geofabrik.de/osmi/?view=areas&lon=152.99843&lat=-27.64484&zoom=15

https://wiki.openstreetmap.org/wiki/Relation:multipolygon

Try instead to use building parts...
https://wiki.openstreetmap.org/wiki/Key:building:part

...or you can just have simple polygons for each part of a building instead of a multipolygon
