Event medals give “+% spell damage for all troops in the current event”. It makes sense that he does extra damage to himself with the medals equipped, no?
No, it’s perfectly reasonable.
You can’t ask that medals work only half way - either they amplify all spell damage (including one done to self - Thrall, Berserker, Penitent, Gar’Nok) or they amplify none and are totally useless.
As others have mentioned, damage is increased overall.
If Thrall’s damage is increased from Medals, then it is intended then even their damage taken is also increased. This is not a bug.