The bug

Witch huts generated by new world generator doesn't spawn witches. Only one pregenerated witch is spawned.

How to reproduce

Example I

Seed & coordinates:

-9144014475768710727
/teleport @s -220 69 20

make some floors and big roof in place of witch hut, after that run several times:

/kill @e[type=!player,type=!witch]

Watch what spawns at the witch hut location.

Example II

Seed & coordinates:

ILoveMojang
/teleport @s 825 68 -863 200 0

Preparation for best conditions:

/fill 832 64 -888 838 71 -896 minecraft:air
/fill 822 73 -868 858 73 -916 minecraft:stone_slab
/fill 832 69 -888 838 69 -896 minecraft:acacia_bark
/fill 832 66 -888 838 66 -896 minecraft:acacia_bark
/fill 832 63 -888 838 63 -896 minecraft:acacia_bark

then repeat:

/kill @e[type=!player,type=!witch]