This KB article explains how interface-specific configuration overrides configuration set for a group.

When an interface is part of a group with a particular knob configured, the interface will inherit the value of that configuration -

user@juniper> show configuration | display set | match group
set groups mtu-default interfaces xe-0/0/0 mtu 9192
set groups mtu-default interfaces xe-0/0/1 mtu 9192
set groups mtu-default interfaces xe-0/0/2 mtu 9192
set groups mtu-default interfaces xe-0/0/3 mtu 9192
set groups mtu-default interfaces xe-0/0/4 mtu 9192
set groups mtu-default interfaces xe-0/0/5 mtu 9192
set apply-groups mtu-default

user@juniper> show interfaces xe-0/0/2 | match mtu
 Link-level type: Ethernet, MTU: 9192, LAN-PHY mode, Link-mode: Full-duplex, Speed: 10Gbps, BPDU Error: None, Loop Detect PDU Error: None, Ethernet-Switching Error: None,

user@juniper> show configuration interfaces xe-0/0/2 | display inheritance
##
## '9192' was inherited from group 'mtu-default'
##
mtu 9192;

If we configure the MTU specifically for the interface, the MTU value that is configured will override the group MTU value. For example,

[edit]
user@juniper# set interfaces xe-0/0/2 mtu 1514

[edit]
user@juniper# commit
configuration check succeeds
commit complete

user@juniper> show interfaces xe-0/0/2 | match mtu
 Link-level type: Ethernet, MTU: 1514, LAN-PHY mode, Link-mode: Full-duplex, Speed: 10Gbps, BPDU Error: None, Loop Detect PDU Error: None, Ethernet-Switching Error: None,

user@juniper> show configuration interfaces xe-0/0/2 | display inheritance
mtu 1514;

This is because the interface-specific configuration will always override any group configuration present. This applies for interface ranges as well- any interface-specific configuration will override configuration set for an interface range.

Please note: Even when interface-specific configuration is set, the group configuration will still include interface although the value does not get applied per the group.

user@juniper> show configuration | display set | match group
set groups mtu-default interfaces xe-0/0/0 mtu 9192
set groups mtu-default interfaces xe-0/0/1 mtu 9192
set groups mtu-default interfaces xe-0/0/2 mtu 9192
set groups mtu-default interfaces xe-0/0/3 mtu 9192
set groups mtu-default interfaces xe-0/0/4 mtu 9192
set groups mtu-default interfaces xe-0/0/5 mtu 9192
set apply-groups mtu-default