Hello,
Next time, please do not erase buildings that have a ref:UrbIS tag.
If there is a situation where you want to create an object that spans over a larger area, just create a container area and make this area the main building.  But do not erase the smaller buildings, just retag them as building:part=yes and keep the ref:UrbIS tag.  This is the unique identifier of every building in Brussels.
Thanks.
I fixed this one.