Hardcore mode does not work properly on servers (allows players to respawn) if force-gamemode is set to true

On a Hardcore server, if force-gamemode in server.properties is true, players who have died and spectate the world will be revived after they join the server again.

To reproduce

1. Start a server with the following settings in server.properties:

   hardcore=true
   force-gamemode=true
   

2. Join the server
3. Die
4. Click "Spectate World"
5. Leave the server and join it again
   → You're back in Survival / Hardcore mode again

Video

https://youtu.be/DIC0TAbnczs