when falling, burning, or drowning in survival in a previously made world in snapshot 19w34a or 19w35a that have now been updated to 19w36a, the player wont take damage. if the world was made without cheats enabled, this seems to be irreversible thus cannot be turned back on.
Thank you for your report!
We're actually already tracking this issue in MC-160464, so I resolved and linked this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature in the future to see if the issue has already been reported.
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