Larian Studios Forums Divinity: Original Sin 2 - The Modding Dungeon Divinity - Original Sin 2 - Modding Troubleshooting Journal Editor - Adding new quests breaks Origins.

Regarding COMPLETION: it doesn't matter how you call it. Journal entries per quest are sorted in descending order by objective priority. The COMPLETION objective is just an empty objective with a very high priority that we assign to the final update of quests, so that
a) that entry always appears on top
b) there is no objective (text) associated with the final entry (since the quest is finished, it can't have an objective any more)

That said, what you're describing indeed suggests that the game is replacing the entire journal of the Origins mod with the one of your own mod. That definitely sounds like a bug. I've told Kevin about it.