Quantum immortality
Max Tegmark now believes that from their own point of view, the person in the thought experiment should not expect immortality. Since they die in some worlds, they afterwards exist in much fewer worlds than they had before. People are less likely to find themselves in a world where their own existence is less likely. Therefore, it is only a possibility, not a certainty, that the person who does the experiment then goes on to feel like they survived. This same problem, of not existing as much afterwards, was pointed out by Lev Vaidman in the Stanford Encyclopedia of Philosophy.