Magma cubes do not spawn in groups

Magma cubes should spawn in groups of 1-4 or 2-5 according to the herd sizes given in the vanilla behavior pack spawn_rules\magma_cube.json. However, they only ever spawn one-at-a-time. According to this comment on MCPE-46540 that is because LavaSlime::checkSpawnRules() cancels spawns that would collide with existing entities, and in Bedrock Edition herds attempt to spawn all at the exact same spot (MCPE-66319).

Steps to reproduce

1. Create a large platform in a biome that spawns magma cubes.

2. Watch.

3. Run /kill @e 3 times and keep watching.

Expected result

Magma cubes spawn in groups of 1-4 or 2-5.

Actual result

Magma cubes always spawn alone.