When a player dies, their death trigger will only work for them and no one else, meaning if the death trigger shows a message or does some other action only the dead player will see it.
This causes desync issues and has already broken a few maps, like max teabag-mmo_arena and maps that use similar systems. I'm guessing this bug came with some unannounced update.
Edit:
I made a temporary fix for this bug using trigger mechanics. It should make it so the player's death trigger is called for everyone, instead of just them. I don't guarantee that it will be perfect, so if you use it and run into any bugs, be sure to let me know.
Link to map: https://www.plazmaburst2.com/?s=9&a=&m=jeje52-dt
Edit2:
The map is unpublished now, as it seems the bug is fixed at last. Thanks Eric!