It seems that combining event box and wiki / shared edits on the same post is technically fragile. It is better to separate them. See Discourse Event time tags disappear sometimes + event box trouble - bug - Discourse Meta and the trouble I had with [CEE Meeting] Introduction to CEE Hub right now (you can check the version history of the first post).
Separating event box from shared edits is a good idea regardless, because with multiple people editing in real-time, chances are that someone will accidentally mess with the event box text format.
When creating an event, we can have the first post with the event box posted by the author, and then a separate post for meeting notes when the organizers require it.