The two spin of series are listed as seasons and multiple times as separate series:
Removed the dupes, but kept the 2 movies which exist on TMDB since they are standalone theater releases.
Jackson Murphy commented
FLCL Progressive and FLCL Alternative are listed as Season 2 and Season 3 of Flcl on tvdb and aired in the US as a TV Series. https://www.thetvdb.com/series/flcl
Like most anime they don't allow separate entries if the admins consider it a continuation of a show already listed.
FLCL Progressive released in theaters in Japan as a Movie.
https://trakt.tv/movies/flcl-progressive-2018-09-28 Links to a working Tmdb Entry.
FLCL Alternative was also released in theaters in Japan as a Movie.
https://www.themoviedb.org/movie/551272 Links to a working Tmdb Entry.
"Progressive" and "Alternative" are movies in (the original) Japan. But aired in the U.S. as series. They're different version.