When using $project, replacing the contents of an existing array with a field will replace the entire array with that field. However, replacing the content of an existing array with a document will replace every element in the array with that document.

> db.collection.aggregate({$project: {_id:0, array:1}})
{ "array" : [ 1, 2, 3 ] }
> db.collection.aggregate({$project: {_id:0, array:"pie" }})
{ "array" : "pie" }
> db.collection.aggregate({$project: {_id:0, array:{snack:"pie"} }})
{ "array" : [ { "snack" : "pie" }, { "snack" : "pie" }, { "snack" : "pie" } ] }

Original Description

As per this post on SO: https://stackoverflow.com/a/52822987/6440033 and this playground:
https://mongoplayground.net/p/wdMj4ZQAHhc there is some unexpected behaviour when attempting to replace the contents of an array field using a nested $map operator.