On the Android version it seems to me that the tone marker that is accepted as the correct answer does not change even in situations where you’d verbally use a different tone. For example 不是 would be written as bù shì even though it would be pronounced as bú shì. And that seems to be the general trend when writing pinyin.
With that said, I suppose it’s probably a mistake then that the accepted tone markers for 不错 are bú cuò. Sure that’s how you’d pronounce it but I dont believe that’s the proper pinyin.