Interesting – yes, I did that in a new subdomain on the same server and the problem didn’t recur there in new content. So I tried using the WP import tool to bring in the page content from the old site, and on the old pages the sidebar override settings still won’t stick.
Also, I went back to the original install, which is a WP site which has been running for about 6 years, but is fully upgraded, and there too newly added pages will take sidebar override settings but old ones won’t.
I guess there must be a database-level issue. Have you any idea what it is? I’d like to find a fix which can save my old page content.