Mám rád tebe. stresses that you like this person and not some other person, which someone probably thought you like instead.
This sentence is spoken by a female speaker. Would then not "Mám tě ráda" be correct?
In the English-to-Czech reverse exercise, "Mám tě ráda" is accepted, since gender is not clear. But, FWIW, the TTS uses a female voice throughout the course.