Steps to reproduce:
1. Create a scheduled event and either wait for it to be recorded, or just put its scheduled time in the past manually in versions where this still works. (I heard there was a patch for that but it still worked in 5.x at the time of this writing.) Anyway, the technical start date of the event should be in the past after this step.
2. Move the bibliographical start date of the event (under the Metadata tab) into the future.
3. Try to change any* meta data field.
4. Maybe refresh.
The meta data changed in step 3 is reverted back to the state it was before after step 2. Additionally, if you watch the Network tab of your preferred browser's developer tools, you should see the response to the request to the `/event/:id/metadata` endpoint created in step 3 come back as a `500`. And if you look into Opencast's log around that time that makes total sense, because there you'll find an exception complaining about the fact that the event has already ended.
The meta data changed in step 3 should be persisted, of course!
I actually only tested this with the start date and the description.