There are those strange formations in at least one Morlovia battle (was doing advanced 17 when I spotted it) where boss is standing behind multiple minions.
When I used Proteus’s special on boss (it should hit boss and adjacent enemies) it affected everything.
I don’t have any other reasonable heroes that hit 3 targets to check whether they also hit everything. But even if it’s only Proteus, it still sounds like a bug (unless that’s a feature related to this particular formation).
Just in case: the particular chain of specials i used is: Wilbur, Boldtusk, Proteus, Sonia - so damage sharing on enemies got dispelled by Sonia. But you can see Proteus’s special on everything. And it’s not just a visual bug, every minion was getting DoT in further turns.