Mobs specific to structures will stop spawning there after upgrading the world

The Bug

Mobs specific spawns from structures such as Witches from Witch Huts & Guardians from Ocean Monuments will stop spawning there after upgrading your world to the current snapshot

Expected Result

The mob spawns from those structures would continue to spawn there

Original Description

So...i upgraded my map and i had a guardian farm...guardians just won't spawn in this version. I created a new map in singleplayter and i had the same result...guardians just won't spawn.