Hi Shirely,
iOS 12.4.6 is the most recent version. It was released on March 20th.
The issue with your Twitter account may be unrelated. It could be an issue with the Twitter app (you can check for updates for the app itself) or the Twitter service. Try logging into Twitter in your browser and see if your tweets are still there. If not, reach out to Twitter support for assistance.
https://help.twitter.com/en/using-twitter/missing-tweets