This ability might be bugged.
I was doing some Hellebron testing as I like to do and when it went off it healed Gotrek AND hellebron.
The only mod I had going was the mod that allows you to give certain lords an item that permanently nets them to keep them from moving. Neither Gotrek nor Hellebron had the modded net item (not avail for gotrek)
Edit: I see the ability is listed to have a miscast explosion is it possible that this miscast explosion heals enemies around Gotrek?