Bisecting can be a good strategy. But you need to look through 100 commits. I need to look through 5-8 PR diffs. Everyone thinks they have the best strategy because they get results with it. Anyway, I'll try your strategy if mine is failing.
Don't forget, a project like Terraform is too big. No one person can know how the whole system works. Trying to look through PRs as a means of debugging is a fools errand. You have to take on a different mindset when working on these large codebases.