A bug in the $rank/$denseRank implementation has been fixed, so that they are consistent with the sort order.

Description of Linked Ticket

Currently, $rank and $denseRank treat null and missing differently, which causes a field to change from null to missing, or vice versa to a rank increment. This is not consistent with $sort behavior, which treats null and missing the same. This causes the rank result to be unstable.

The SBE implementation currently copies the Classic semantics. When we update the semantics, we should update in both engines.