Following T335971, it appears the extension is saving preferences for temporary users.
Update the code to treat temporary users like anon users.
To help with searching:
In PHP preferences are saved via UserOptionsManager::saveOptions
In JS preferences are saved via methods defined on options.js: https://gerrit.wikimedia.org/g/mediawiki/core/+/809d4c9a9dd2cff6321cdae7a41e75b6c362cbbd/resources/src/mediawiki.api/options.js
Code Search: https://codesearch.wmcloud.org/deployed/?q=UserOptionsManager
Wherever these do something different based on whether a user is anonymous or registered, they may need updating (including comments).
Tests and comments should also be updated.