Probably because the duo course (along with most courses) teach a type of Welsh that more closely resembles the spoken language. As a result formal dictionaries for native speakers rarely include the "new-age" forms of words and instead use the traditional form. In this case it would be eithaf.
bug in duo? i get the little green speach-bubble with a 2 in it, but when i tap the bubble this page shows 'no comments'.
I am told there is a typo in my answer "Ydw, dw i'n gallu canu yn eithaf da." For sure, "eithaf" should not be considered a typo ...
I have just checked and it was in the database as an accepted answer. If it rejected again, please add another comment here with a cut-and-paste of the answer that you typed in.