Edit: I read again and now I understand other thing. Are you asking why your progression is not 100%? Or are you asking if you need to complete entire campaign on Insane to “overwrite” the 85.99%?

Did you play on the first months? Also, there is (was) a glitch that the game changed difficulties without notice and you can complete checkpoints playing on Intermediate even when you were playing on Insane (Playing on insane achievement is not affected if you completed some checkpoints on Intermediate).

It is a really rare glitch, I don’t know how to trigger it but it happened to me on Act 3 Chapter 2 two times.

Check if every chapter is completed on Insane, and usually if you only need one chapter or two, you can just play that chapter and that’s all. You don’t need to play entire campaign again.

Hope this helps