TMDB no longer has the Horror category under Genre for TV Shows.
As per their guidelines, Horror should be added as a Keywords.
For some shows like Constantine and Stranger Things, the "Horror" keyword was added but it is not being added on the show's genre on Trakt.
This issues should be looked into. The ability to reflect the Horror keyword on the website.
Another issue that should be resolved is the the fact that Action & Adventure are basically one tv show genre on TMDB. Same as Si-Fi and Fantasy. There should be a way where we could pick one or the other to be reflected on trakt instead of both.
An update will be out for this tomorrow and will start looking at the keywords horror, romance, history, music, and thriller — and assign Trakt genres.
As for the double categories, there isn’t an easy way to just pick one on Trakt which is why we assign both.
Hamza Behbehani commented
Romance, History, Music and Thriller can also only be added in TV Show keyword and not genres.