This is by design in Odoo. Fields of type fields.Html are meant to be displayed as read-only in the list (tree) view and editable in the form view. This is because the HTML content can contain complex formatting and might not be easily editable in the list view. To ensure that the HTML content is displayed properly, it is displayed as read-only in the list view and can be edited in the form view.

If you want to make the field editable in the list view, you can use a different field type, such as fields.Text, which can be edited in both views. Keep in mind that the formatting options for a fields.Text field are more limited than those for a fields.Html field.