This is article explains how MX304 behaves when you have one MIC and you installed it in Slot 1
If you have one MIC and you have installed it in MIC 1 slot instead of MIC 0 slot, we may observe some undesired behavior, like in customer case DHCPv6 was not working.
Its recommended, If you are installing one LMIC, we recommend you to plug it in the LMIC0 slot If you are installing two LMICs, we recommend you use slots LMIC0 and LMIC1.
https://www.juniper.net/documentation/us/en/hardware/mx304/topics/topic-map/mx304-maintaining-interface-modules.html