Buildings from ways and multipolygons are loaded in table `osm_building_polygon`. But a table for `osm_building_multipolygon` is also loaded, the content is not used except to ensure an `osm_id` is from a multipolygon. To check if the object is from a multipolygon we have only to check if `osm_id` is negative. It is the counter part of e0c8ece375/layers/building/building.sql (L89)
I checked the objects are the same after this change.
building
Docs
Read the layer documentation at http://openmaptiles.org/schema#building

