I'm using enhanced biomes, which adds its own dirt and tilled earth variants, and flax cant be planted on it. Other modded crops can do so.

Confirmed

After looking at the code, there's not much we can do. This is something that the dev of Enhanced Biomes is doing wrong:

We check if the soil is 'fertile'. Eventhough the soil IS fertile(showing) we cannot detect this codewise.
My suggestion would be to take this to the dev of Enhanced Biomes.