This is not really an errorenous pronounciation. The â in đầu is pronounced as /ə/ (like a in about, american accent) and a in giàu is /a/ (like a in father, american accent). I hear that clearly as /a/.
That word is also tricky because both "gi" and "d" is pronounced same in the northern accent. Duolingo doesn't make this any better, because it uses sometimes southern words and but pronounce them with northern accent.
Duolingo is bad for people who wants to communicate with people living in southern vietnam.