The problem is with the Postman API.
I’m also facing the issues with the PUT of collections.
From my experience this happens cause Postman API changes the collection after uploading in a way that it may not be valid according to the schema anymore.
Small example: use a description with an empty string. After upload the description now has a value of null. As this is no valid description according to the schema you get a malformed json error.
There are also some fields added by Postman itself, which are not described in the schema and also causes the schema validation to fail.