Datagen Error: Couldn't define tag as it is missing following references:

I'm working in the forge mdk for 1.20.2 and using Terrablender (version 3.2.0.11) to create a custom biome. The biome itself works and generates just fine, but I need to create a custom tag for it in order to have a very specific structures generate in that biome and ONLY that biome. That's when the problem comes in: I've created a ModBiomeTagGenerator class that extends BiomeTagGenerator from the vanilla code, and the same way the vanilla BiomeTagGenerator assigns tags to biomes, I'm trying to assign my custom tag to my custom biome so I can use it in a has_structure json. However, when I do this and try to runData, an error pops up: "Couldn't define tag decayingplanetmod:is_abandoned_city as it is missing following references: decayingplanetmod:abandoned_city." After experimenting, I found that I can add my tag "is_abandoned_city" to other vanilla biomes, but I can't add vanilla tags or ANY tags to my custom biome, abandoned_city, so it's definitely a problem with the custom biome itself, despite the fact that the biome generates in-game just fine. I've included the code that defines my biome, my biome tags, and assigns my tag to my biome. I've scoured online for similar problems and found very few-- the ones I did find had no concrete solutions, just "it suddenly started working" or "I updated to another version and it works now," which is very vague, as they don't specify what they're updating or how. Nothing seems to work so far.

Faulty code: https://paste.ee/p/79my3

Any input is appreciated.