Darren Branford

I am experiencing this in my world, too. The guardians do seem to be getting stuck on the sides of the drop chute and are seemingly able to travel vertically at will. 

I have also also observed that the guardians lose the ability to cling to the walls whenever they attempt to shoot their laser beams. 

Played on an iPad. 

I can confirm this (was going to file my own ticket before I found this one).

In my case, I built the tower in my single player survival world before the latest update, with every platform being made out of top slabs. It worked extremely well.

After the update, not a single mob will spawn on it.

Having the same issue on iOS and Android devices. Guardians no longer spawn in worlds created before 1.2.

However, they are spawning as normal in new worlds created in 1.2

EDIT: After further testing, it seems that only Monuments discovered before the update are affected. I have searched for a new Monument in one of my old worlds and it is also spawning Guardians as normal.