To my experience the ports are not channelized per default - but have experienced this behaviour with cheap QSFP DAC cables from China. When using a properly coded cable, no issue occured.
If you intend to use the QSFP ports for Virtual Chassis, you need to convert them to vcp-ports. See this guide: https://www.juniper.net/documentation/en_US/junos/topics/task/configuration/virtual-chassis-ex4600-cli.html . Especially step 9, but I would recommend also reading the rest to get a good understanding.