It’s happened to me twice this season both times when I enter a public game where there is a GR already open and peeps are killing mobs inside. It lasts for 15-30 seconds then I stop gaining xp. But I also got short massacre bonuses of 20-35 mobs both times that awarded 5 Billion xp or so
Whilst you were waiting for them to finish the GR, did you go to another map and kill some stuff, clicked a pylon / shrine, then come back to town and the clones were killing stuff whilst you were in town?