In ticket SERVER-31225 it is suggested that mongo should be set to "type=forking" and this has been applied globally to all mongo deployments. However, it is not impossible that in your /etc/mongod.conf you can have :

processManagement:
  fork: false

And then your server starts and after a while systemd notices it hasn't forked and kills it.
I think startup=simple (or leave it to default) should be the default in the release and if people want to customise their own startup type, the RPM package should not overwrite customised systemd scripts.