Ref T13559. If you create comments A and B, then delete comments A and B, then undo the deletion of A, the UI undoes the deletion of B instead.
This is becasue the undo rows are shipped down with a static scalar metadata reference. When copied multiple times to create multiple undo rows, they reference the same data object.
Preventing this in the general case is a problem with greater scope. For now, just avoid rendering these rows with any metadata so they don't alias a single data object.