1. Your Editor preferences should NOT be "Plain text area".
2. Create a Database activity.
3. You could choose a preset such as "Resources" but above all you must have a template in List view with a <table> element in Header.
4. Add an entry.
5. Display List view (everything should be as attended).
6. Go to Templates.
7. Select "List view template".
8. Just click in Header content, don't need to modify anything and Save.
9. Return to List view and see table deformed.

The editor try to correct what it supposed to be an error (non closed <table>) and it add a </table> in the end of th Header.

This is not really a bug but a unexpected behavior.
See in french discution : https://moodle.org/mod/forum/discuss.php?d=452742