Players can revive in Hardcore mode by choosing to clear player data when exporting the world.

How to reproduce:
1.Create a world in Hardcore mode
2.Die
3.Export the world
4.Select "Clear player data"
5.Export
6.Import the world
7.Enter the world

What I think is the reason for the error:
In this process, you don’t need to edit AppData or use third-party tools to modify the world. You only need to use the functions of vanilla minecraft to achieve it.

Possible solutions:
Disable Hardcore mode
Clear player data when exporting world

Note:
The issue reported by this ticket has been resolved, but this ticket is not marked as resolved.

May be: one of the versions Preview1.21.10.23、Preview1.21.10.24、Preview1.21.20.21