Second run, this time via Ouro. Got all 3 "assets" on the final mission, Dreck spawned at the end of the map - killed him and his mob (did lots of damage) and mission ended. Still no bonus mission. I did not go back to the lab once the mission ended.
First run, got 2 "assets" on the first trip through the map, brought them back, went back out, got the 3rd asset, brought them back, ambushes spawned, Dreck killed at top of ramp, mission ended, no bonus mission. So, zero-for-two on Brass Tax.
I've read through all of the content here and everywhere else I've found tips, nothing is working. Is this a known bug?