Unholy Might cannot apply Wither with spells.

As the title says.
It can be applied with attacks, and I was able to confirm the Wither visual effect,
However, no matter how many times I hit enemies with spells while Unholy Might was active, neither my friend nor I could confirm it.

Unholy Might says “you have a 25% chance to Wither on hit,” so it shouldn’t matter whether it’s a spell or an attack, right?