SharePoint online : announcement design issue

We use the "announcement" list in SharePoint online. When I open it, I can see the announcement list, everything is.

But when I open one announcement and see the detail page, the design will be change.

・menu on top will be invisible (user cannot go back to top page)

・user must click "See more" to show the full content

(Please see screenshot for more details.)

How can I change the design?