I have seen this happen off and on for a while now. It may not be a bug, but it has happened on many different characters and in different areas of the game.

First you use his "Deepest Cut" attack and apply some marks on the target and gain an extra turn.
Next, attack the same target again. With a perfect attack, the target should take additional damage from the mark exploding.

What you see happen is this:

Target takes damage on perfect hit
target gains a significant amount of health
mark explodes and deals additional damage.

The health gain in the middle causes the mark damage to be irrelevant.

If you use a different character to perfect hit and trigger the marks, there is no life gain.