Currently, $merge stage doesn't give a consistent result when the incoming document has dollar fields in it. For example, with (whenMatched: 'replace', whenNotMatched: 'insert') strategy, it may either trigger a DollarPrefixedFieldName error, or a FailedToParse error depending on whether the update is performed locally or remotely through an update command. The stage may also work for certain strategies like (whenMatched: 'keepExisting', whenNotMatched: 'insert').

We should come up with a way to return consistent results when there're dollar field documents in $merge.