The idea is that if you have a bug that you've covered with something like a system test of acceptance test, and you can figure out how to cause the bug with something lower in the testing-concept-test-pyramid, you should.
I eventually find the bug, and I'm able to identify the change that needs to be made to fix it. Most devs I've worked with over the years stop there. They have a failing test and a commit that fixes it. Job done. Time to move on. I think this is a missed opportunity. It's hard for me to imagine a bug that can't also be reproduced via a unit test or an integration test. What I prefer to do is to add a second failing test at a lower level on the pyramid, and then make sure that the "fix" I'm applying fixes both the higher-level acceptance test and the lower-level test.