Digging into this a bit more I confirmed the following:
BUG-11333 was supposedly fixed in the "Maintenance Dawa" release on February 3rd, 2021, per the release notes here: https://releasenotes.secondlife.com/viewer/6.4.12.555248.html
Firestorm merged the "Maintenance Dawa" code as of version FS version 6.4.13.63251, per these release notes: https://wiki.firestormviewer.org/changelog:firestorm_change_log_6.4.13.63251?s[]=dawa
Since we're seeing the issue in Firestorm 6.5.3 we can assume the bug was possibly not fixed after all, or a more thorough fix needs to be implemented. It seems odd to me that the code on either the viewer or server side would even allow such powerful object alterations sim-wide without the user explicitly going into edit mode with the objects, or a user script directly pushing the changes.
@Linden Lab What's the best way to get this looked at again? Could we have BUG-11333 re-opened or should we file a a new bug in the Jira? I think an issue which can randomly corrupt the visual experience of an entire region for all users should be a pretty high priority to resolve.