Unaccounted for damage

Sometimes when im playing league, one of my friends or myself will die and the cause of death will be somewhere between 100-400 damage that had no discernible cause. one very notable instance was this: (im providing as much detail as i can remember) Team fight bot lane between the area around the inner turret and the inhib turret. my friend was playing sona and was on the run, jhin ults and hits sona twice, sona dodges 2 shots, including the 4th shot and should be in the clear with about 300 current hp at level 16ish. suddenly, without any animations going off, they take the final 300 damage killing them out of basically nowhere. there was no way jhin could have done the damage, the rest of the enemies had no way to kill her where she was. The zed item couldnt have been to blame either, she hadnt taken any auto attack damage in well over 4-6 seconds.

we had 3 of us watching intently and none of us could reason a source for the damage. I even looked at the replay using the replay HUD app. after reviewing the footage, there was still no reason for 300 damage to have just happened. The death recap claimed jhins ult did the damage, but i know for a FACT that isnt the case.

There have been other instances of this happening in my games, but none are nearly as notable for me to give a description of the situation.

not sure what is causing this damage, but it happens every once in a while not sure what the cause of this random damage is but it does affect games when it shows up and it is a problem.

(also, sometimes when im trying to last hit, it seems like minions semi frequently take a lot more damage than the pattern of damage ticks prior and are killed before i can auto them for the cs, this might just be me not calculating the damage of the cannon minions properly, but i dont think thats the case... less important though)

unfortunately i dont have the footage from replay HUD anymore, sorry