Not to say that you don't review your own work, but it's good practice for others (or at least one other person) to review it/QA it as well.
When you merge them into one it's usually a cost saving measure accepting that quality control will take a hit.
I've been coding an app with the help of AI. At first it created some pretty awful unit tests and then over time, as more tests were created, it got better and better at creating tests. What I noticed was that AI would use the context from the tests to create valid output. When I'd find bugs it created, and have AI fix the bugs (with more tests), it would then do it the right way. So it actually was validating the invalid output because it could rely on other behaviors in the tests to find its own issues.
The project is now at the point that I've pretty much stopped writing the tests myself. I'm sure it isn't perfect, but it feels pretty comprehensive at 693 tests. Feel free to look at the code yourself [0].
[0] https://github.com/OrangeJuiceExtension/OrangeJuice/actions/...
When it comes to code review, though, it can be a good idea to pit multiple models against each other. I've relied on that trick from day 1.