Subdirectory installations changing address

(This has happened on at least 8 different sites I manage, not all of them using the latest version.)

Usually, when setting up a site, I will install WP in a subdirectory, eg “/website”, and then change the Site Address (URL) to the domain root, following the instructions on here: https://codex.ww.wp.xz.cn/Giving_WordPress_Its_Own_Directory

However I have noticed that on a number of sites, the Site Address (URL) box has reverted from “example.com” back to “example.com/website”. This breaks a number of internal / external links, as all pages bar the home page now have the subdirectory inserted in the URL – eg, if someone was linking to “example.com/johnsmith” that link wouldn’t work now; it would need to be “example.com/website/johnsmith” to work – and it is also messing with the SEO for a number of sites.

I didn’t change it, and the clients didn’t change it either. I don’t think it has come from any particular upgrades, either. Any ideas why it would happen, and what I can do to prevent it happening again?

thanks in advance!