RE: https://docs.mongodb.org/manual/reference/operator/update/currentDate/

$currentDate does not provide any assurance that consecutive updates will show an increasing (or even static) value. It is possible for an external client to perform consecutive reads where a field being updated using $currentDate is observed to go backwards.

This effect can theoretically occur on a standalone server without any changes to the system clock. Although the opportunity for the effect increases with a replica-set (elections) or system clock changes.