I don't know why players are going invincible like that. Does that happen in vanilla LMS aswell?
Well the invincibility, as far as I can tell, is normal. At the very least, I can say that the game has behaved this way since I began playing it. In Vanilla LMS, after a round ends, a countdown to the next round begins during which all the players respawn. For the duration of the countdown, the respawned players are invincible and so the only person who can take damage is the winner of the previous round (unless it was a draw). Once the new round begins, players become vulnerable again as the map resets.
An interesting note about the post-round invulnerability is that if an invulnerable player activates a damage-reducing or damage-nullifying effect, they will be vulnerable again once the effect wears off. So if you are invulnerable, then fire the Skull Barrier, you will be vulnerable once more when it wears off.
The problem I'm having is that the invulnerability
does not wear off when the map resets. I've done some testing and managed to produce this without Chaos Generator active, so I am thinking its a V3 problem.
Another interesting note is that it seems to affect exactly half of the players in the game at once. For example, I tried a game with 31 bots and the result was that 15 of them, plus myself, retained invulnerability - then I tried with 63 bots and 31 of them, plus myself, retained invulnerability.