Spawn rule "population_control": "ambient" does not work

The population_control "ambient" does not work.

If I copy the pig spawn rule, change "population_control" to "ambient" and weight to 500, no pigs will spawn.

"population_control": "ambient" does work for vanilla bats, so there seems to be something hardcoded or bugged.

Sample spawn rule:
 

{   "format_version": "1.8.0",   "minecraft:spawn_rules": {     "description": {       "identifier": "minecraft:pig",       "population_control": "ambient"     },     "conditions": [       {         "minecraft:spawns_on_surface": {},         "minecraft:spawns_on_block_filter": "minecraft:grass",         "minecraft:brightness_filter": {           "min": 7,           "max": 15,           "adjust_for_weather": false         },         "minecraft:weight": {           "default": 500         },         "minecraft:herd": {           "min_size": 1,           "max_size": 3         },         "minecraft:biome_filter": {           "any_of": [             {               "test": "has_biome_tag",               "operator": "==",               "value": "animal"             },             {               "test": "has_biome_tag",               "operator": "==",               "value": "cherry_grove"             }           ]         }       }     ]   } }