Hi guys, it might be a little later reply but still.
I had the mission and did not had problem with intercepting the Tanyas group.
Simply point over the group once the red cross appears and let my groupd directly intercept them. I have ntoiced that if you are too far than you might cross over them without intercepting. Simply mnoving up close and then clicking the red cross triggers the ambush allways
. Did you automaticaly entered sector or did u clicked on the 'enter sector' buton?
The guys there did not atacked me, so i placed my men on the highgrounds and rained 40mm grenades on the groups. Fast fraging. I becamed enemy of Tanya afterwards. Basicaly the quest was labeled as completed once u kill the special NPCs in the encounter.
cya