Magma Cubes spawn too much in Nether Wastes and Basalt Deltas. I'm not sure about other regions.
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Comments 4
It literally says in the 20w15a changelog that magma cubes spawn "very frequently" in basalt deltas, so that's not a bug.
Though I haven't been able to reproduce for Nether wastes. They seem to spawn as much as before the Nether update.
Magma cubes are supposed to spawn more in Basalt Deltas, but I don't know about the Nether Wastes.