So if your code is ascertained to work at the high level, you also know that it must be working at the lower level too.

I have 100% seen bugs that cancel each other out; code that's just plain wrong at the lower level, coming together by chance to work at the higher level such that one or more integration tests pass. When one piece of that lower level code then gets fixed, either deliberately or because of a library update or hardware improvement or some other change that should have nothing to do with the functionality, and the top level integration tests starts failing, it can be so painful to figure it out.

I've also seen bugs that cancel either other out to make one integration test pass, but don't cancel each other out such that other integration tests fail. That can be a mindmelt; surely if THIS test works, then ALL THIS low level code must be correct, but simultaneously if THAT test fails, then ALL THIS low level code is NOT correct. At which point, people start wishing they had lower level tests.