mongoimport --mode merge issue with multiple level JSON objects

Question

Looking at the following example from the MongoDB documentation: https://docs.mongodb.com/manual/reference/program/mongoimport/#ex-mongoimport-merge, the --mode merge works for 1 level deep collections.

I'm trying to merge collections that are n-level deep and the levels after the 1 level get overwritten as in using the --mode upsert flag.

Is that a bug in the merge function or was it not intended to work recursively?

Thanks!