Runic magic

Here's the situation. The description of the Magic Ward rune states that "on the next attack, the unit will steal a random positive effect from the attacked enemy unit." However, Berserkers attacking a unit under the control of the Dark Magic spell stole the control effect. But this (control) is not a positive effect. After that, I cast the control spell on the same unit again and gained control of two enemy units.
Anyone who understands the mechanics, please explain.