This is why semver is not a silver bullet. It only covers what the project maintainers think are breaking changes. Their unit tests don't consist of all program

This is why semver is not a silver bullet. It only covers what the project maintainers think are breaking changes. Their unit tests don't consist of all programs that use their code. When you use undocumented behavior, is it correct to blame the project and take no blame for yourself? That seems to be what happened here.