Thanks for your hint. I checked it on the ground. Actually the address of K26 building is Neue Mainzer Straße 28. There ist no address Kaiserstraße 26 on the ground. Btw address Neue Mainzer Straße 26 is on the other side of Kaiserstraße.

I merged both K26 parts to one OSM building with 2 OSM building parts. See https://www.openstreetmap.org/changeset/152862348