I put it as future update because there wasn't an option to put version 1.21.13 only 1.21.11
The update was 04:42 AM GMT time
What shows in my windows event viewer is:
AnalysisSymbol
Rechecking 0
ReportId 53ab8cb9-cbfb-40e7-9868-174b650f8678
ReportStatus 268435456
HashedBucket 9a0c15f3b6efe1b3f6d5d5e21b50bc6e
CabGuid 0
I hope this helps, thank you
The affected world folder was too large to attach directly, so I uploaded it to Google Drive:
https://drive.google.com/file/d/1QMO2FtLSXeCXmFx6hZT6cFUCOkRUxe_-/view?usp=sharing
This is the affected world after it was opened in Minecraft Java Edition 26.3 Snapshot 7 and the underground base area became void.
The link is set to “Anyone with the link can view”.