The $position operator ($push/$pushAll) has a restriction that ignores all positions past the end of an array:

If the value of <num> is greater or equal to the length of the array, the $position modifier has no effect and $push adds elements to the end of the array.

https://docs.mongodb.com/manual/reference/operator/update/position/#position

This seems intentional in the original design (SERVER-2363) but limits common use cases. It also differs from behavior elsewhere using dot notation on an existing array:

> db.foo.insertOne({ _id: "d", field: [] });
{ "acknowledged" : true, "insertedId" : "d" }
> db.foo.updateOne({ _id: "d" }, { $set: { "field.4": 5 } }, { upsert: 1 })
{ "acknowledged" : true, "matchedCount" : 1, "modifiedCount" : 1 }
> db.foo.find(}
{ "_id" : "d", "field" : [ null, null, null, null, 5 ] }

It would seem logical, beneficial, and consistent to enable the positional operator to add the values of $each to any position of the array. This provides additional value for upserts by providing an alternative to dot notation that may create inconsistent documents (SERVER-13478).