there is one sutta that asserts ananda was sotapanna but when appraising the vast majority of ananda's episodes in the suttas it appears doubtful his mind was sotapanna sotapanna is enlightenment and yes it is sudden. the moment the mind can genuinely let go, stream entry happens but after it happens, we do not want to then run around advertising "i am enlightened" because "i am" is the contrary to enlightenment