Below URLs may be helpful for you. Are you using routing instances?
https://kb.juniper.net/InfoCenter/index?page=content&id=KB28735&actp=RSS
https://forums.juniper.net/t5/Junos/SNMP-issue/td-p/14989
https://forums.juniper.net/t5/Junos/SNMPD-AUTH-FAILURE/td-p/273550