Hai! That just depends on the context, of course. In Indonesian, it is possible to mix these pronouns in many situations. And I always wanted to know why. But I think, first, it is easy to understand the informal use of both pronouns when the relation is intimate, between children, close friends or in a very close relationship. When the context is formal, even the same relationship (when other adults are present) would use formal addreses (Ibu, Bapak, Mbak, Mas, Kak, Saudara, Saudari, Adik, even the proper name or surname is combined or not with the term of address in the place of the pronoun), most of the time between adults, and perhaps one adult can decide to avoid the formal pronoun when addressing to a child, but always using Saya in the conversation. Still, I think I cannot explain it so well because I am not a native, but I know a simple example for the Duolingo sentence (using Saya / kamu) is when a teacher talks to a young student or teenager. Then, in the conversation, the teacher use the pronoun Saya, but would avoid using Anda to address the student, even more when the student is very young; perhaps to not be extremely formal or to not treat (ironically) children like adults, and also that use cannot mean to be disrespectful in any way. The student would use both Saya / Anda to not be disrespectful, and I think it would be better not to change Saya to Aku, or Anda to Kamu for any reason. Then, I think it is always the "authority" (or between authorities, directors, enterpreuners) that keep the formal address, and can mix the informal address, not for oneself, but for others.
I would also like to hear some native voices (I know the uses can change in different regions of Indonesia), and perhaps they can add more options for formal and informal conversations.
Here is a link with some of the uses I have mentioned above: https://blogs.transparent.com/indonesian/indonesian-personal-pronouns-kata-ganti-orang/
And please, it is better not just to keep the simple formula "Saya = formal, Aku = informal" (I agree), and "Anda = formal, Kamu = informal" (not always); in the second person, the formula could also be, "Anda = polite, very formal", and "Kamu = polite, familiar, informal", so this cannot be always so clear.
Other good readings about Indonesian pronouns:
Selamat belajar! :)