password spontaneously changing

have you checked your site for malware/hacking: https://sitecheck.sucuri.net/

That site finds nothing, but I’m not sure it truly supports a multisite installation or that it can dig into all the plugins.

You can also try Installing the plugin Wordfence plugin and do full scan of your site.

Ironically, I have this plugin, but never used it until today. It turns out I did have malware installed. Thank you for your help.