Slimes do not spawn at the same rate as magma cubes when spawned from monster spawners.

While creating the mod https://modrinth.com/mod/no-peaceful-despawn I discovered a vanilla bug.
Slimes do not seem to account for not being in slime chunks when spawned in from monster spawner blocks.

Yarn mappings have this

ChunkPos chunkPos = new ChunkPos(pos);
boolean bl = ChunkRandom.getSlimeRandom(chunkPos.x, chunkPos.z, ((StructureWorldAccess)world).getSeed(), 987234911L).nextInt(10) == 0;
if (random.nextInt(10) == 0 && bl && pos.getY() < 40) {
    return canMobSpawn(type, world, spawnReason, pos, random);
}

I went back to confirm this is a bug in vanilla by playing an unmodified version of minecraft and created a test environment in superflat.

How do reproduce the bug:
Create a creative superflat void world with cheats in any version of minecraft such as 1.21.8
Split the spawn platform in half using glass
Place monster spawners down equal distance to the player., one will have magma cubes, and one will have slimes.
Turn the difficulty to hard to increase mob spawns
Turn the time to midnight.
Notice half of the platform has many more magma cubes than slimes.