When using an addon that adds more biomes (i've tested bioplentia. expansive biomes, and end expansion) and enabling the toglles in the world settings, it won't modify, nor create any new biome-related features in the overworld, however, it will work for:

• Structures (custom structures will still be able to generate)
• Certain custom trees (the trees that were programmed to generate in existing biomes will generate, but the trees made for the new biomes won't generate)
• Entities (any new entity made for existing biomes will generate in those. they won't generate in new ones)

It won't work for:

• Custom biomes
• Entities made for those specific biomes
• Structures made for those specific biomes
• Features (such as trees) made for those specific biomes

To emulate this bug just download the end expansion addon and the expansive biomes/bioplentia addon and it will be found that new biomes and features will work only for the end dimension, but not for the overworld.

If you find this report, please comment your experience or upvote it, as this makes addon making much harder if not impossible, thanks for your time.