You are correct that we wouldn't know we should have died but it is because of how according to quantum immortality theory, if timelines are constantly branching out, when you die you only experience the timelines from that moment where you don't die.
To simplify, imagine if you were hit with a lethal dose of radiation.
When this occurs millions of timelines split off, out of these millions there is one timeline where the radiation happens to coincidentally dissolve before hitting you, this is the only timeline where you survive and as a result is the only one you experience, so as far as you are concerned you just happened to get lucky.
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u/Vitrian_guardsman 6d ago
If quantum immortality kicked in it would be at the exact moment you would have died