In some occasions there is a 4 sections cube hole in tha mansion, where there also is no floor generated
See pictures for more details!
Where I found this (16w39b):
Seed: 3888442674554197564
Location: -25121 | 80 | 17617
MC-107818
Mansion generates with a 4 section room missing
Resolved
Fixed
4
Community Consensus
Minecraft 16w39a, Minecraft 16w39b, Minecraft 16w39c, Minecraft 16w40a, Minecraft 16w41a
Minecraft 16w43a
Retrieved
I can confirm this, and that it was also present in 16w39a.