With Version 1.01 of the FAQs came out, Fantasy Flight answered the question about the victory conditions on Round 8.
Q: In “Under Siege,” if the Imperials secure the 4th capture
point at the end of Round 8, which “End of Mission” conditiontriggers, the end of Round 8 or the Imperial player securing4 capture points?A: Since they happen at the same time, the Imperial playerdecides (see “Conflicts in a Campaign” in the RulesReference Guide).