Well, sometimes a fix makes you <facepalm>. This is one of those times. Heres your explanation.
All of the voxel data was being saved across multiple databases when it should have been going to a single database. When we tried to retrieve it after a server outage we were getting none of the voxel data because it wasnt saved correctly.
Each time we outaged the servers (several times) in the last few days, the data became more and more permanently mish-mashed, thus becoming impossible to parse correctly.
So the truth is
that data is gone. However, now that this issue is fixed, any future changes you make will finally (at last!) be saved correctly and you should lose no more voxel data.
Were pretty darned confident this change is correct. We have tested this change by making edits, then outaging the servers again internally, then starting the world again to verify that the voxel changes are there. They are.
This really, really should be fixed. We ran internally for over four months without ANY voxel data being lost. Its only this new server configuration that was new, and now that this is fixed, the issue shouldnt happen any longer. (The problem in this case was a SINGLE setting. By fixing that one setting, all the other problems that cascaded from it are also resolved.)