From what I understand, when XCP-ng reverts to a snapshot it restores the full VM state from that point (metadata included, not just the disk contents) so the creation date field would get rolled back along with everything else; that might be why it now matches the snapshot timestamp rather than
the original. I might be wrong about the internals though.
It's a bit confusing if you were relying on that field to track VM history, and I don't think https://docs.xen-orchestra.com/xo5/manage_infrastructure#snapshot-management covers this explicitly.
Might be worth a mention to @Team-Documentation-Knowledge-Management; it's the kind of thing that catches people off guard because nothing warns you upfront that metadata rolls back too.
My $0.02.
