https://www.cnet.com/news/twitter-admits-bug-exposed-some-android-users-protected-tweets-for-years/

Those using the iOS version of the app or Twitter for web weren't impacted.

Twitter fixed the issue on Jan. 14, but isn't sure how many accounts were affected, so it's recommended that you double check your settings if you're an Android user and want your tweets protected. The company is contacting people it knows were affected and reactivated the protected tweets setting for them.