Mist is pushing by default a source-address for protocols such as NTP. Typically, the main IRB configured in the UI.

For example:

set groups top system ntp source-address <default-ip>

If you simply try adding a different source-address, the following conflict error will show:

"error commit-confirm: Only one source address per family allowed in ntp for default routing-instance"

As a customer you might require to source this traffic with a different address.

Currently there's no option to add an NTP source-address via the UI, so the default needs to be deleted and the new one needs to be added manually via additional CLI commands.

This change was made intentionally recently and now protocols such as RADIUS, SNMP, NTP, etc.. are getting a default source-address pushed. 

This works per design and if you wish to use any other source-ip, the only way is to delete that default one and add the desired source IP manually. E.g:

delete groups top system ntp source-address <default-ip>

set system ntp source-address <desired-ip>