If you have a Python module named after a built-in, for example, io.py and try and test a function from it, then the debugger in VS Code (at time of writing) will consider it to be third-party code, and won't respect its breakpoints.
See Here for a discussion on this.
A solution is to enable the A005 Ruff rule with lint.flake8-builtins.strict-checking = true.