Chassis reboot warning is printed in boot messages every time after 'disable-lag-enhanced' is configured even if the 'disable-lag-enhanced' feature has actually taken effect.

Warning message when system is rebooted:

...
Validating against /config/juniper.conf.gz
[edit chassis aggregated-devices]
'disable-lag-enhanced'
warning:  Chassis configuration for lag enhanced has changed. A system reboot is mandatory. Please reboot *ALL* routing engines once commit is complete.
mgd: commit complete
Validation succeeded
...

The subjected warning message is always printed when the 'disable-lag-enhanced' knob is configured. Even if the system had been rebooted after committing the knob. Upon every reboot, you will see that warning message printed. It is safe to ignore the warning if the system has been rebooted after it was configured. 

The below command can be used to confirm 'disable-lag-enhanced' feature has enabled or not:

user@router> start shell user user
user@router:~ # sysctl -a | grep disable_lag_enhanced
net.disable_lag_enhanced: 1                  >>> Lag Enhanced Feature is disabled 

user@router> start shell user user 
user@router:~ # sysctl -a | grep disable_lag_enhanced
net.disable_lag_enhanced: 0                  >>> LAG Enhanced feature is now enabled