I have previously been able to restore a db to mongo v4.0 without issues, but when I restore to v4.2 I see auth errors. In both cases the db does not have auth enabled. This is the error when restoring to mongo 4.2:

$ mongorestore -h [HOSTNAME] -p 27017 -d [DBNAME] --sslCAFile [PATH_TO_CERTS_DIR]/ca/mongo-ca.crt --sslPEMKeyFile [PATH_TO_CERTS_DIR]/client/mongo-client.pem --ssl /tmp/tmp.qq5fLSuatw
2019-10-06T03:59:55.045+0100 error connecting to host: could not connect to server: connection() : auth error: sasl conversation error: unable to authenticate using mechanism "SCRAM-SHA-1": (AuthenticationFailed) Authentication failed.Connection to [HOSTNAME] closed.

$ cat /etc/mongod.conf | grep auth

1. authorization: enabled

This is the exact mongo version:

$ mongo --version
MongoDB shell version v4.2.0
git version: a4b751dcf51dd249c5865812b390cfd1c0129c30
OpenSSL version: OpenSSL 1.0.2g 1 Mar 2016
allocator: tcmalloc
modules: none
build environment:
distmod: ubuntu1604
distarch: x86_64
target_arch: x86_64
$ mongorestore --version
mongorestore version: r4.2.0
git version: a4b751dcf51dd249c5865812b390cfd1c0129c30
Go version: go1.12.4
os: linux
arch: amd64
compiler: gc

I read the release notes for 4.2 but did not find any mention of something changing in the auth system.