Can confirm 24w33a
Issue persists in 1.21 Pre-Release 3 and it manifests in the behavior of both Magma Cubes and Slimes. When adequate space is available for a size 0 slime to spawn, all naturally occurring slimes of varying sizes will instead spawn at that location.
Here's a step-by-step guide to reproduce the undesired behavior:
Launch a super flat world in Minecraft version 1.21 Pre-release 3.
Locate a designated slime chunk within the world.
Summon end crystals in the designated area, positioning them precisely 0.52 blocks above the ground.
Move between 24 and 128 blocks away from the slime chunk.
Observe the spawning of slimes of sizes 0, 1, and 3 within the specified area
For further investigation, repeat the process with the end crystals positioned 0.5 blocks above the terrain surface. Note the absence of slime spawns within the designated region when the end crystals are placed at this height.
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Can confirm 24w35a