You use Elementor to build your website. Use it to edit the affected page and check whether it contains an Elementor widget for HTML code. Because what is currently visible in the frontend is only in an iframe, which embeds another page (which apparently doesn’t even exist). If you remove the widget, this output should also be gone and you should be able to store your text again via Elementor.
You could also check whether a revision of the page with your text is available. You should find the revisions outside of Elementor when editing the page with the block editor on the right-hand side. Also go to the backend, under Pages, e.g. the Privacy Policy page and click on “Edit” – not on “Edit with Elementor”. Then look on the right.
I would also recommend that you do a malware scan with WordFence – or a security plugin that you already use. This change cannot just happen.
Thanks very much for the comprehensive response.
I took the actions you suggested and the issue is now resolved.
I’m also running WordFence to try and identify source of the random code.