I cant say for sure whats wrong then. Only reason they shouldn't be getting roll window is because killed unit too far away...
If you have more info how this happens exactly (what mob booster is looting f.e) I can try to resolve it.
Do you mean those shadowy things on last boss?
If so... Do you know whether noobs had killed theirs also? Boss cant be attacked if there are even 1 of them alive.
And did booster stay "afk" after that phase ended since all of them will eventually fade away?
1: It happens with all bosses
2: I think it's before the second boss,when you run in and have to kill ,i think 3 of them ( they are already fighting these pandas), after you kill these groups,then the panda Boss appears.