One way to think of this: Uncertainty (at least on this level) is in the observer, not the coin. It comes up heads or comes up tails, with 100% chance of the thing that actually happens.
Before the flip, you assign 50% to each outcome, but that’s your uncertainty, not the coin’s, and the result may as well be secretly predetermined by the universe.
After you’ve seen 20 heads, that part is now probability 100% (it’s knowledge, not uncertainty, on your part), and the next flip is still 50⁄50 (to your knowledge, presuming you have reason to trust the coin and not update toward an unfair flipper).
One way to think of this: Uncertainty (at least on this level) is in the observer, not the coin. It comes up heads or comes up tails, with 100% chance of the thing that actually happens.
Before the flip, you assign 50% to each outcome, but that’s your uncertainty, not the coin’s, and the result may as well be secretly predetermined by the universe. After you’ve seen 20 heads, that part is now probability 100% (it’s knowledge, not uncertainty, on your part), and the next flip is still 50⁄50 (to your knowledge, presuming you have reason to trust the coin and not update toward an unfair flipper).