Steps to reproduce:
- Make sure damage numbers are enabled in options
- Inflict Violent Heart Attack upon enemies, I tried a gathering of gremlins on depth 1 and some scuttlers on depth 2
- Kill one of them
- Observe the explosion to and note the damage number

Expected result:
- The amount of damage surrounding enemies take is displayed

Actual result:
- Surrounding enemies take damage, but the amount doesn't appear

Damage numbers from weapons appear as expected, so this seems localized to the skill in question.