Issue: Editing section opens wrong section

This issue has often occurred in vw3 as well. It seems to me that it might be much preferable to store each section separately.

It depends on the article - as we've come to know it depends on the article contents and is usually the result of broken WYSIWYG tags. But as you've noted, and is one of the strong reasons for such a feature, this problem would not exist if sections were actually stored separately rather than implied by the editor contents. However, it would be a significant change, so we will not see it until 4.1.

If you can provide more information about the problematic article, I can see if additional workarounds need to be implemented to prevent broken tags from interfering with detection.

Yes, it would be a big and welcome change. It would be a good one to implement for 4.1
An issue would be to prevent the wrong content being conversion to the new method of storage. That could get messy.
Maybe add a function to check for any doubtful section tags before conversion.

Marking as a Duplicate, since this is due to a design flaw that cannot be fixed without implementing the following feature: https://www.vaultwiki.org/issues/3259/
It would automatically be fixed by implementing said feature, so there is no need to leave a separate report with Open status.