According to a tweet from Twitter’s official support page on Wednesday, updates will roll out over the next two months to ensure tweets don’t disappear. Twitter said the issue seems to happen most often when the timeline auto-refreshes.
“The background: a tweet would move up the timeline as replies were added to the ongoing convo. Since some convos can evolve quickly, this made it so you didn't see the same tweet repeated in the TL,” Twitter Support said.
The good news is that this “disappearing tweet” incident only happens if you’re using the Twitter iOS or Android app. However, if you’re reading a long thread about something and a new tweet comes in as a reply, you end up losing your place, which is super frustrating.