> Running the suite in CI on every build would have caught a lot of this

yeah, provided:

  1. test cases are being designed to cover functionality
  2. test cases are implemented correctly - i.e. they actually fail if the implementation is defective
  3. the tests aren't all commented out
  4. the test suite fails if a test fails
  5. CI actually detects test suite failure
  6. failing tests block changes from being merged (apart from initial triage to stabilise the suite)

Five years ago, outsourcing your software development (including QA) to the lowest bidder might have gotten you results like this -- if you didn't explicitly require automated tests as a deliverable, you wouldn't get any, or if you got some they'd be vacuous and not test anything.