Custom biomes (in add-ons) do not generate in v1.18.0 even with creation of custom biomes enabled.
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In Changelog 1.18.0 ( bedrock ) the "Known Issues" section is written:
New world generation in this release is currently incompatible with the 'Creation of Custom Biomes' experimental toggle. Expect worlds with custom biome generation to be potentially unstable and for custom biomes to only exist in currently saved out areas of the world.