This problem is a bug with the MeetingManager Edtior script. This problem happens when you have the button event open while running the app.
The temporary solution for now is to minimise the event before hitting run.
Event Open looks like this:
Event Closed looks like this:
Issue appears to stem from the scenario type being branching instead of meeting. Once a new meeting scene was created the event branching worked and no extra buttons were displayed at the end of the dialog.
The extra button line appearing at the end of the play through continues.
Update on multiplying buttons auto-populating scenes:
I recently built a scenario that included branching via buttons. I made sure to create the scenario as a meeting scene and compiled my dialog lines and buttons in the inspector, as usual. Once I was done creating the scene, I hit "prepare for build" and went into VR for UX testing. In VR, I saw some tweaks that needed to be made, made them, hit "prepare to build" and went into VR again. I did this a handful of times. Each time I did, the number of buttons in the scene grew and eventually even began appearing in the mirror scene of a different scenario.
Photo reference found here: https://imgur.com/WQwkcXM
From the Inspector, I could see an increasing number of button fields appearing in the original meeting scene (note: not in the inspector of the aforementioned mirror scene). I would remove these button fields from the inspector and they would simply reappear and grow once I hit 'Prepare for build' and went into VR again.
Inspector with auto-populating button fields seen here: https://imgur.com/mIxt1g3