I mean the damage which applies DH should not occur on the same cast as the actual execute. It should be either damage + DH or execute alone.
If you think about how the logic for this ability should be, it should be like this: First you determine if there should be normal damage or execute. Then you apply the chosen effect. If the chosen effect is damage then you get DH. If the chosen effect is execute then it shouldn't really matter.
But apparently how it works is, it first applies damage, then it checks to see if it should execute. This produced the correct behavior before, but now with DH it doesn't.
or IDK maybe it doesn't apply damage first. Maybe DH just triggers because the ability is flagged as a damaging ability even in cases where it doesn't deal regular physical damage. Like I know that the execute is technically implemented with damage but by the game mechanics definition it isn't "damage".