Hi there,
Welcome to the Atlassian Community!
And I'm sorry to hear you've experienced this issue.
Basically, this can happen if you have an unstable internet connection because Trello needs you to stay online to be able to update your data. Have you noticed any issues with your connection when this happened?
Also, there might be some software such as firewall, proxy, antivirus, or VPN that cause disruptions to Trello's performance and prevent your data to be sent to Trello servers to be properly saved.
You can try going through the instructions in this article to help you recover any previous card description that was saved.
If this doesn't solve the problem, I encourage you to submit a ticket at https://trello.com/contact and provide some additional information (such as links to affected cards; were you using any Trello app when the issue occurred or only your browser, etc.) so we could look further into this.
Hope this information helps!
Alina | The Trello Team
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