Previously, the headings hierarchy was placed also in the Document sidebar, in the "Table of Contents" panel:

This panel has been removed so the only way to be aware of the incorrect headings level is to intentionally check the document outline tool in the top bar.

I'd like to propose to add a notice, even a small one, directly in the Heading block sidebar so users will be immediately informed the hierarchy is not correct while they set the heading level. An option might be placing the notice under the buttons in the sidebar: