It happens more likely if you're in the endstone box with obsidion floor spawn. If you place any blocks on the end spawn, all of that will be deleted after you die and reenter the end. It's probably because the spawn resets every time someone enters the end.
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Thank you for your report!
We're actually already tracking this issue in MCPE-52483, so I resolved and linked this ticket as a duplicate.
That ticket has already been resolved, which means this is not considered a bug and won't be fixed. Please do not leave a comment on the linked ticket.
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