So what does the "Dogma of the Faith will always be preserved" mean?
Nobody knows what it really means because that statement is part of a longer sentence that is cut off. The true context is lost except for the Church's "brass" who were told all of the secrets and full message.
That's why I don't pay attention to the Fatima Center or any other Fatima enterprise (that's essentially what it is) that revolves around speculation about the Secrets and message. It's become like a cult, too. These cultists place Fatima over the Apocalypse of St. John.
What is concretely told is good enough for me, and it's obviously come to pass and still happening - the world wars, diseases, Marxism spreading across the globe, apostasy in the Church, etc.