Thanks for the feedback, everyone.
The problem was indeed that the combat was triggered from the Prop. Here is the line of events that DOESN'T work:
Prop STEP: conversation > conversationX
conversationX > encounterY
encounterY OnEndCombatIBScript > conversationZ
What DOES work:
triggerW > conversationX
conversationX > encounterY
encounterY OnEndCombatIBScript > conversationZ
I'll probably continue to use the IBScript to trigger a post-encounter conversation because if the encounter was optional, based on choices in the preceding conversation, I'd like to not necessarily be locked into the next conversation lined up in that trigger.