Roost restores half of the user's maximum HP. If the user is a Flying-type, it will lose its Flying-type (thus being treated as a single-type Pokémon) until the next turn. If a non-Flying-type uses Roost, then there are no side-effects.
So, yeah its not a glitch. Non flying type Pokemon do not become grounded or lose their "flying type" because they dont have one to begin with. This will only happen with actual flying type Pokemon.