The reason plugins in the mu-plugins folder can’t be activated in the plugins page is because that’s not how mu-plugins works. The mu stands for must use.
Improtant things to know:
mu-pluginsload beforeplugins- WP loads PHP files in the
mu-pluginsfolder- it does not search subfolders
- it does not look for plugin headers
- PHP files in the
mu-pluginsfolder have no activation status, they are always loaded.- this is true for all sites in a multisite, there is no conditional loading.
So code in the mu-plugins folder will never have an activate/deactivate link. You can never put code in both mu-plugins and plugins. You also can’t just put a plugin in mu-plugins, it needs a PHP file that includes the plugins main file to work.
mu-plugins is a folder of PHP files, where WP loads all the PHP files before it loads the plugins or themes.