Hi,author,I am a Chinese player that my English may be not so good.

I tried to let your mob spawn in modded biomes,so I wrote the_config list[mobs.frostmaw.biome_config] like this:
#Allow spawns in these biomes regardless of the biome tag settings
biome_whitelist = ["theabyss:waywardsea"]

And for sure,I add the modded biome into the has_structure file,like this:(the bold words are what I added)
{
"//NOTE": "Any biome you want Mowzie's Mobs structures to spawn in must appear here. By default, this includes all overworld biomes and tags. Modded biomes may not by included by default. During placement, biomes are restricted by the dictionary tags from the Mowzie's Mobs config.",
"replace": false,
"values": [
"minecraft:the_void",
"minecraft:plains",
"minecraft:sunflower_plains",
"minecraft:snowy_plains",
"minecraft:ice_spikes",
"minecraft:desert",
"minecraft:swamp",
"minecraft:forest",
"minecraft:flower_forest",
"minecraft:birch_forest",
"minecraft:dark_forest",
"minecraft:old_growth_birch_forest",
"minecraft:old_growth_pine_taiga",
"minecraft:old_growth_spruce_taiga",
"minecraft:taiga",
"minecraft:snowy_taiga",
"minecraft:savanna",
"minecraft:savanna_plateau",
"minecraft:windswept_hills",
"minecraft:windswept_gravelly_hills",
"minecraft:windswept_forest",
"minecraft:windswept_savanna",
"minecraft:jungle",
"minecraft:sparse_jungle",
"minecraft:bamboo_jungle",
"minecraft:badlands",
"minecraft:eroded_badlands",
"minecraft:wooded_badlands",
"minecraft:meadow",
"minecraft:grove",
"minecraft:snowy_slopes",
"minecraft:frozen_peaks",
"minecraft:jagged_peaks",
"minecraft:stony_peaks",
"minecraft:river",
"minecraft:frozen_river",
"minecraft:beach",
"minecraft:snowy_beach",
"minecraft:stony_shore",
"minecraft:warm_ocean",
"minecraft:lukewarm_ocean",
"minecraft:deep_lukewarm_ocean",
"minecraft:ocean",
"minecraft:deep_ocean",
"minecraft:cold_ocean",
"minecraft:deep_cold_ocean",
"minecraft:frozen_ocean",
"minecraft:deep_frozen_ocean",
"minecraft:mushroom_fields",
"minecraft:dripstone_caves",
"minecraft:lush_caves",
"theabyss:waywardsea",
"#minecraft:is_taiga",
"#minecraft:is_river",
"#minecraft:is_ocean",
"#minecraft:is_mountain",
"#minecraft:is_jungle",
"#minecraft:is_hill",
"#minecraft:is_forest",
"#minecraft:is_deep_ocean",
"#minecraft:is_beach",
"#minecraft:is_badlands",
{
"id": "#forge:is_overworld",
"required": false
}
]
}
I tried to switch the "required" as true or false in has_structure file,and tried to wrote biome id into biome_tags = [] in config file,but it didn't work in both situation.

Looks like you did that correctly. But you may need to alter some other settings in the frostmaw config such as the height limit.

I altered the height limit as -64 to 256,but it still don't work :(

Frostmaw structures cannot generate in water. That could be the issue. Is this biome a water biome?

Sorry for I shelve for so long because I'm busy recently.For your question,no,this biome is a snowy biome like snowy plains.I also add Umvuthi generate in the aether,but it still didn't work.I really don't know how to make it.

Can you please upload your whole config file?

This is the config file,I also added has_mowzie_structure.json into it.The other related mods are "The Abyss: Chapter II" and "The Aether".If you need,I can send you my modpack.
config.zip