The NTP client (SRX) will always build the association from the default routing table (inet.0); instead of the custom routing-instance table.
The reason for not supporting this feature is during the device boot-up, the NTP update might fail; so it will not be able to set the date/time on the device, if the custom routing table is not populated at that time.
This might lead to syslogs showing the incorrect time stamp in the log messages, till the time NTPD finds the route to reach the server and learn the time properly. If we change the NTP client to look-up in the user defined custom routing-instance table, then no route/path will be there to reach the NTP server in the default table.
Use only the default routing table and source the NTP update traffic from the default routing table.