Reposting this from the Java Support forum because it didn't fit very well there.
At the start of quarantine, I started a skyblock server with some friends. I downloaded the map from this site, updated to 1.15.2, and everything was fine and dandy. About a week or so ago, I updated the server to 16-pre2 to get all the Fancy New Nether Stuff, which greatly improved our skyblock experience.
Among the things I tried to do after the update was find a turtle egg. I found a beach, built a little sand area, and waited around 30 blocks away for a long time. Nothing spawned. I've copied the world into creative, and expanded the sand area, added grass, and two water pools. Tested on both 16-pre2 and 16-pre5. I've linked a screenshot of the little island I made. (The sand is in a beach biome, the grass is in a forest/plains biome, and the water is all in ocean/beach biomes.) What I've found is that the water mobs spawn like crazy, but not a single land animal spawns.
According to the wiki, there is no reason that land animals, most notably turtles, shouldn't spawn. Is this a bug? Or am I doing something wrong in building the island specifically to spawn animals?
Actually I just tested in a 1.16 pre 2 all beach world and I'm not seeing any turtle spawns, except for the ones that generated with the world, if I use the /kill command to remove them they don't seem to be respawning.
The beach is at Y=65. And sea level is Y=63 (I flew to the mainland to check).
And it's a personal server that I run from my computer. There was nobody online except me while I was testing it.
There are however 30 villagers, 10 bees, a few cats and iron golems, and well over 40 chickens back on the main island. But the island is ~500 blocks away from my turtle spawning rig so I'd just figured those chunks would unload and wouldn't effect the mob cap on the turtle island.
And I checked again on 1.16rc, and I think you're right about the general turtle spawn. Nothing spawned on land except monsters in the caves. If it's not fixed in the next snapshot, I might report it because this would be a Bad bug to release 1.16 with.