Okay, next round.
I've taken out all my races but one (no rejections), then added a cave race. They didn't have any non-sentient castes, and they were using the following crop tags:
[INDOOR_FARMING]
[OUTDOOR_FARMING]
[INDOOR_GADENS]
[INDOOR_ORCHARDS]
They were causing rejections.
I've added all the remaining outdoor tags to them, and there were no rejections.
So... What actually causes this? It can't be the lack of edible underground plants. I have those. One is edible raw:
[PLANT:MUSHROOM_BROWNCAP]
[NAME:brown cap][NAME_PLURAL:brown caps][ADJ:brown cap]
[USE_MATERIAL_TEMPLATE:STRUCTURAL:STRUCTURAL_PLANT_TEMPLATE]
[MATERIAL_VALUE:1]
[EDIBLE_VERMIN]
[EDIBLE_RAW]
[EDIBLE_COOKED]
[MATERIAL_REACTION_PRODUCT:SEED_MAT:LOCAL_PLANT_MAT:SEED]
[BASIC_MAT:LOCAL_PLANT_MAT:STRUCTURAL]
[GROWDUR:400][VALUE:1]
[USE_MATERIAL_TEMPLATE:SEED:SEED_TEMPLATE]
[MATERIAL_VALUE:1]
[EDIBLE_VERMIN]
[EDIBLE_COOKED]
[SEED:brown cap spawn:brown cap spawn:4:0:1:LOCAL_PLANT_MAT:SEED]
[SPRING][SUMMER][AUTUMN][WINTER]
[FREQUENCY:100]
[CLUSTERSIZE:5]
[PREFSTRING:rounded tops]
[WET][DRY]
[BIOME:SUBTERRANEAN_WATER]
[UNDERGROUND_DEPTH:1:3]
[PICKED_TILE:3][DEAD_PICKED_TILE:182]
[SHRUB_TILE:28][DEAD_SHRUB_TILE:28]
[PICKED_COLOR:6:0:0]
[SHRUB_COLOR:6:0:0][DEAD_SHRUB_COLOR:7:0:0]
There are also other underground plants that are either edible_cooked, or have edible growths. And even then, they had outdoor farming previously!
I'm kind of unsure.