As far as I recall, the issue appeared in both cases.
Sometimes the start date wasn’t updated, and sometimes the end date was affected. I didn’t have a chance to properly reproduce it as it’s really time-consuming and intricate.
I submitted a tiny PR which fixed it to some extent. We are using that as a helper plugin for now.
The current workaround is also:
Edit post and remove the event BB code, save the post
Edit the post, re-add the event BB code with the desired date, and save post
This re-writes the event dates from scratch into the topic.