That is the problem of the game. The effect of Maddening Song does not match with the description.
https://divinityoriginalsin2.wiki.fextralife.com/Maddening+Song

Sing, causing characters in your target area to become Mad. Maddened characters will attack those nearest them, regardless of alignment. (Maddening Song now targets only enemy characters as of Patch v3.0.168.526)