[SpicyLemonZest]

In many contexts, there's just no such thing as a safe behavior change which should be allowed without a specific decision from you to allow it. As a database systems guy, I've seen countless examples of customer breakages caused by a developer's decision that some behavior or another is so trivial it doesn't need to be tested.

When you're working on developing a random utility function (real example!), it's easy to say "come on, it's no big deal to return DECIMAL(14, 4) instead of DECIMAL(12, 3)". It feels like they're basically the same, updating the test is make-work, and the guidelines saying you must document it as a breaking change are pointless annoyances. It's hard, requiring substantial amounts of knowledge and expertise, to recognize that this change will cause a production outage because the schema of a customer's view is no longer write-compatible with their existing data.

[EamonnMR]

In your story though the hapless dev just changed the test. And the reviewers approved it.

This suggests that there are so many changes to tests that it's just become background noise.

[SpicyLemonZest]

It had, and that's precisely because of the lack of anything like the expect() tests described in the OP. It's laborious to reliably scan through a big test diff and identify when it's describing a user-facing change, and people are inevitably going to autopilot through it. If you have a golden file (the standard name in my area for an equivalent mechanism to expect() tests), the reviewer's work is a lot simpler: any non-append-only diff is a breaking change and must be either fixed or communicated broadly before deploying it.