Hi, the museum is mapped as a multipolygon but it is done in a way that is invalid for a multipolygon in osm. Outer parts cannot join each other. We have a wiki  on multipolygons that explains this.

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

I suggest that you delete the multipolygon and just map as two individual buildings and the the central part should be mapped as a roof instead of a building as it is just a covered walkway linking the two buildings.

Another way to map the museum is to tag each bit as a building part and then add another polygon (tracing over the outside nodes) around the entire outside of the three parts and tagging that as a building with the museum tags.
https://wiki.openstreetmap.org/wiki/Key:building:part
Building part method is mostly used for complex 3d buildings but is also useful for outlining parts of simpler buildings.
Unfortunately the individual buildings parts are not rendered in the layers available at osm.org, only the surrounding shape. 

Another method is to just trace one line around the museum and tag that.

