BoP adds dirt variants such as sandy dirt.

The green thumb skill causes constant grass tall-grass and flower growth on these dirts.

Also happens with Midnight's 'Midnight Grass'

not a bug, it's how the trait is implemented.
Currently there is no plan to fix the 1.12 behaviour, it'll most likely be weeded out by the rewrite in 1.15/1.16 that's planned.