Only teleporting 1 person in the fireteam during the timestream event is not a lag issue - it is a code issue. It is literally the game choosing 1 person when the design intends for 3. The "fixed" this before when they made the decision by the game random instead of the people furthest back - they could fix this one as well but choose not to because such a design ruins a run rather than help it.
If they start banning people for cheesing Crota, then what about all the people who cheesed the Templar, or Atheon?
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To me that sounds as if the game only registered one person being in the fight. Or the other two intended teleports suffered packet loss.