I find that it does? Against Knuckledragger (BAR on, no UCP):
One point of Unf0rseen hits for 612k.
If I tap him with melee to apply Death Mark first, I get 1102k (80% increase as expected)
Add five points of Ambush (20%)
One point of Unf0rseen hits for 734k
Tapped for Death Mark, Unf0rseen hits for 1322k (80% buff)
Using Execute to apply Death Mark also has Unf0rseen hit for 1322k (in case there were questions about Execute “closing” the Ambush bonus, since this ends Decepti0n; these staggered enemies apparently don’t re-acquire you for a brief moment, though it’s easily enough time to get a quick shot off that includes the Ambush bonus). In the shot below, you can see the Ambush marker (from some recent research) circled in red. It “hits” low, and during combat, there’s usually a lot of noise going on around the point of impact (not the least of which is the huge shock detonation, so it’s tricky to see).
What I did find was that the damage is not buffed by Decepti0n - no matter how long we’re “under”, the damage is the same (which makes a kind of sense… by definition, Unf0rseen is triggered by the termination of Decepti0n), but still - so many things don’t make “sense” that I had to test to confirm for myself.
My build for this testing and proper peer review (using a fire Bone and Legendary Hunter COM):
Next test: does barrel damage get the Ambush bonus? Stay tuned while Adabiviak goes insane trying to standardize that test!