This one room (minecraft:woodland_mansion/1x1_b5.nbt) in woodland mansions is impossible to spawn from what I can tell. I am a modder and I was going through the vanilla woodland mansion code to see what rooms spawn where in mansion. Then I noticed that the second/third floor runs this method to grab regular 1x1 rooms for those floors. In WoodlandMansionPieces$SecondFloorRoomCollection class, this method came up:

public String get1x1(Random random) {  
   return "1x1_b" + (random.nextInt(4) + 1); 
}

The random here only returns a value between 0 and 3 and the +1 makes the range 1 to 4. This means "1x1_b5" is impossible to be chosen. The room looks like this: 

After talking with other modders, we went through the code some more and it turns out, this issue persisted since 1.11.0 when Woodland mansions were added. I also went through the other rooms and no other room has this issue. All other rooms can spawn except for "1x1_b5". (There has been two reports before by the same guy about missing rooms but they listed a bunch of rooms that do spawn so it seems better to make this a separate report about the rooms that are truly not spawning with code samples as proof)