After playing on my familys newly-updated-to-1.19 Realms and logging off for about an hour, I noticed that I had lost at least two hours of progress from before I had logged off. It was entirely rolled back, and the most recent backup did not include the progress. It seems like this issue has been reported by several other people on things such as reddit as well.
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Thank you for your report!
We're tracking this issue in MC-252921, so this ticket is being resolved and linked as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
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