It’s not that, its the fact that only one entity can have a waypoint at a time.
When you try to do trigger → relay (all) → waypoint, you are saying put that one waypoint on every person. Since the rule is one entity per waypoint, the game (or if you’re cheeky, the waypoint), puts the arrow on the last player to join the game, or at least that’s what my tests showed. It’s a small thing, but still a thing.