Skritter is meant to always apply tone changes to 不. If it doesn’t, there’s something wrong and we’d appreciate if users help us out by reporting it! In the case of 不錯, though, it seems that everything is as it should, which is true for 不是 as well. When I bring these words up on my account, they are both written with bú, which is how it’s pronounced. I already linked to this blog post in the reply to your other question, but I’ll post it here to in case someone sees this but not the other one: