Animals not spawning after adding grass to completely mobless superflat worlds.

I'm not seeing any passive mobs spawning in empty 1.13.2 single player creative worlds, one with quartz blocks up to level 74 one with just a single bedrock layer at the bottom. I used /fill to place grass blocks and flew up 50 blocks or so and waited an hour with no spawns at all.

Both worlds had plains biomes, the second world was newly created, no way there could have been any animals anywhere.

First world had daylight cycle deactivated, second was bedrock except for the grass which was lit by torches so no mobs, passive or hostile.

In 1.12.2 they popped in almost immediately

Render distance 15 chunks on all three.