This is by design. In most dictionaries, you will see 一 written as a first tone, even in words where it’s never pronounced with a fist tone (which would be most words). In an educational context, such as Skritter, we are a little bit more free and can do whatever we think is most helpful for students. In the case of 一 and 不, we have decided to show tone changes in words that are pronounced with a different tone, which is true in your cases (it’s always pronounced as shown in Skritter, even though what you wrote next to it will show up in dictionaries). If you want to read more about this, please check this blog post:
Good luck with your studying!