Shabsai Tzvi was also a huge talmud chacham, who was declared Mashiach by many leading Rabbonim. He also supposedly performed miracles, some of which are mentioned by big Rabbonim as well. R’ Bender mentions one in his sefer on the Parsha. Yet we all know that Shabsai Tzvi was a fraud.
The Rebbe was the mashiach sheker of our times. Just like with Tzvi, even after his passing, people are still hanging on, hoping that it’s true. But just like with Tzvi, it’s not true.
One of the reasons that the Gra was opposed to Chassidus was because he foresaw this happening. It just proves him right.