If something is considered to be true, but something is not true, is sentence ‘something is considered to be true‘ not true?
The phrase is considered to be reports what the speaker believes to be a general opinion. It does not necessarily mean the speaker agrees.
Christmas is considered to be the happiest time of the year but I think it is the summer solstice.
would be a possible way of showing how it might be used. Of course the speaker may agree as well, it does not necessarily imply they do not.