likalaruku: 18w44a is a Java Edition snapshot. This report is for the Bedrock edition, is closed, and is a duplicate of another report which is also closed. Please try searching again. The reports for Java Edition are all named MC-####. If you can't find a match, make a new report by clicking the red Create button at the top.

Having this problem in 18w44a. I have a Superflat world where houses have spawned on top of leaves, & the villagers are spawning in ridiculous populations inside the leaves.

Although I did a search before creating this report, I didn't go back far enough to find MCPE-14467. This appears to be a duplicate of that report.

I built a test jig consisting of a hollow, covered 1-chunk area down to level 15 with a stack of oak logs in the middle and oak leaves filling the layer around the top log (level 34). As I watched, a spider spawned right on the leaves, nowhere near the log. I also regularly get spiders, skeletons, and zombie villagers on the top of my cactus farm, which is all glass blocks.

More: A little later I was back in my Survival world, on my roof looking at the Zombie Villager tree. I noticed a spider spawned in the leaves just below where the Villager had been in my Creative copy (pic #1). When the day came I pillared up to take a look and found that it was actually in the leaves, with no air blocks at all as you can see In pic #2. Pic #3 is looking at it from above. Note that the leaves are 2 layers thick there. Pic #4 is the same angle but after I removed the top layer. Pic #5 shows the view from below, with the spider still in the remaining layer. So it must have spawned right in the leaf blocks, with air blocks below it.