I can see that they are "unregistered" and then I loose their trail …
You’ve found the right place: there are two instances of the plugin switcher, one for themes and one for languages.
At a high level, each time a plugin is switched in or out (e.g. as the result of the theme or language being changed), the system rebuilds the cache of shadow tiddlers by re-reading the tiddlers from each of the active plugins. So, when we switch from one theme to another the shadow tiddlers from the old theme are not included in the new cache of shadow tiddlers.
Best wishes
Jeremy.