Starting in Junos OS release 17.1R1, you can send untagged traffic without a native VLAN ID to the remote end of the network. To do this, remove the native VLAN ID from the untagged traffic configuration by setting the no-native-vlan-insert statement under inerface configuration hirarchy. If you do not configure this statement, the native VLAN ID is added to the untagged traffic. But there is an exception, ACX series do not support this option, this is worked per desgin. 

There is a warning message simply indicated the command is unsupported, it applied for ACX series box (JUNOS or EVO).

user@router# show interfaces et-0/0/8 

flexible-vlan-tagging;

native-vlan-id 1;

##

## Warning: statement ignored: unsupported platform (ACX7100-32C)

##

no-native-vlan-insert;

As per Juniper PR 1708563, Juniper do not support no-native-vlan-insert in ACX series boxes (JUNOS or EVO) as of now.