They spawn on top half slabs but not bottom half slabs. Also they spawn on carpet. Slabs won't keep light out so if you want to make a dark spawning platform you need to use whole blocks covered with bottom half slabs that stick out a 16 block chunk past your spawning platform. Spawning isn't actually broken, it just works according to certain rules that many bedrock players think are not very good.
The only sure way to make sure nothing is spawning is to make a perimeter down to bedrock and slab the whole thing. My witch hut perimeter actually produces pretty well. It’s a lot of work without tnt duping flying machines like in java, but it’s the only way to be sure that you aren’t missing cave spawns.
Watch GruvaGuy's explanation of spawning in bedrock. It clears up the basics.
https://www.youtube.com/watch?v=srAL6EpwucQ
I can confirm that the update didn't fix it for xbox one. Very frustrating. I have the exact same problems as previous posters.
The Bedrock spawning/despawning does actually make sense. It's just a different logic than Java. It means you can't watch a Java youtube video on how to build a gold farm and make it work in your bedrock world. It's kind of fun to have to work things out for yourself a bit more. I've made a great (Gruva's) enderman xp farm, a witch farm, Guardian farm, pigman farm, and mob farm. It's totally possible!