This answer is about as useful as a chocolate teapot 😂
Obviously I do whatever I can to try to not to let the battery run out. Not great to have a bug that makes it a nightmare to charge the thing again if it happens though.
I was sceptical of this myself, though it worked for me on S20 ultra (oneUi 3) can't verify for other devices . Turn off the phone and plug it in to a generic slow charge brick. The phone should then display it at 0% but quickly shoot up to whatever percent your actually on. Then turn the phone on and low and behold it worked. https://youtu.be/SH7jIyyU9pQ tip from this YouTube video (step 3)
Edit: as soon as you've done this you can plug your fast charger back in
I have this problem every time my phone is updated. It is 100% a software bug from Samsung as my phone is never near water or high humidity when it happens. The steps below are the ONLY way I've found to solve it, but it works every time. If you've gone through this thread and are still having problems, TRY THIS!
1. Make sure charger is unplugged from phone, but still plugged into the wall.
2. Press and hold Bixby+Power+Vol Down to enter maintenence boot mode.
3. Release all three when the screen goes black.
4. When menu appears, select boot normally by pressing and HOLDING the Bixby button. Phone will boot. Continue holding Bixby.
5. When you see the Samsung logo (Samsung only, not Samsung Galaxy with Android), plug in the charger. Continue holding Bixby button.
6. After boot is complete, it may still show the moisture detected warning. Click OK.
7. Release Bixby button
The warning should go away and your phone should be good until the next time Samsung decides to send an update.