Is test automation on CI/CD really trustable?

I was running a project with test automation on CI/CD and suddenly I got a question.

Is test automation on CI/CD really trustable?

The reason why I get this question is below.

For example, let's assume that I edit some codes on branch A and other codes on branch B and I make PRs.

As I made a test automation, The codes on branch A and branch B would be tested automatically and let's say two tests are passed.

However what if do the two parts of edited codes have no merge conflicts but interact each other and make bugs?

Then how could I trust the test automation and click the merge button?