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The reason is that the biome doesn't have any normal grass associated to it. It does have wormy tendrils and eyeballs, but as this area isn't evil those don't apply.
I would guess that the RNG happened to not select any of the available kinds of grass for this biome, something that's probably very uncommon.
Either DF accepts very rare freak cases like this or it includes a check to ensure at least one kind of grass is available in every biome. It could be that there is such a check, but that it's broken by the introduction of spreading evil, as evil used to be tied to regions, so a region was all evil or all non evil (or all good), so eyeballs alone would be sufficient to satisfy an "any grass?" check. If so, the logic would have to be extended to check that there's any neutral grass available in regions that are neutral natively. |
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