On our 1.13.2 SMP world we are having an issue with very low mob spawn rates. I have gone around in spectator mode looking at 4 individual afk players and found a max of about 5 total mobs in unlit caves around them with no more new ones spawning anywhere.
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I'm not seeing any passive mobs spawning in 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.
In 1.12.2 they popped in almost immediately
Render distance 15 chunks on all three.
I'll post this separately if there isn't already a better bug report but I thought it might be related.
There is a mob cap, maybe those afk players or a lot of things going on in the spawn chunks are causing the server to reach it.
Have you tried running /kill @e(warning, this kills players too) and making sure there are no farms in the spawn chunks?