If it's not documented it's not part of the "public API". Software using Semantic Versioning MUST declare a public API. This API could be declared in the code

If it's not documented it's not part of the "public API". Software using Semantic Versioning MUST declare a public API. This API could be declared in the code itself or exist strictly in documentation. However it is done, it should be precise and comprehensive. These can both be considered fixes. Undocumented functionality is not part of a public API.