Some of deepslate ores never generate in splotches of deepslates

I see no deepslate coal ores in splotches of deepslates, though diamond ores, redstone ores and iron ores generate in deepslate style.

Instructions:

1. Open Minecraft snapshot 21w16a.
2. Create a new world and enter the seed 1480304068.
3. It must be a default world type.
4. /tp 181 15 -78
5. You will see no deepslate coal ores.

Expected Results: 
Deepslate coal ores should generate in splotches of deepslates.

Observed Results: 
Deepslate coal ores don't.