There is an issue causing joining players to hang indefinitely at the island loading screen, forcing players to restart the game when trying to load into the island.
Edit #3 - issue have been found, took me 24h and 50 private versions to isolate it - but i believe i found it.
The issue was caused by the following
Island Settings:
Join in progress: Spawn During new round
Override spawn immunity time: set to 0 seconds
The island has two locations, the spawn zone and main arena
Players spawn in the spawn zone where the player spawners are located
after a few seconds, the active players are teleported into the main arena
and a damage zone is enabled over the “spawn zone” to ensure no players are left behind
since v39.00 - joining players during ongoing rounds are eliminated by the damage zone, even though they should not spawn while round is in progress
And for some reason get stuck at the island loading screen, freezing fortnite, forcing game restart - critical issue
Setting the Override spawn immunity time to value greater than 0, seem to resolve the issue