Welcome to OpenStreetMap.

Unfortunately, the buildings which you merged do not actually overlap and the process of merging lost information about one of the buildings ( https://www.openstreetmap.org/way/601848558/history )

You should be able to see the gap between the buildings in the Bing street side imagery here
https://www.bing.com/maps?cp=51.537625%7E-0.042093&lvl=21.1&mo=om.1&pi=-8.7&style=x&dir=333.2