Hi,
You said:
"From the SRX we can ping the syslog server using a routing instance."
is this instance different than the default routing instance?
If so, try configure log sending, to be sourced from interface in default routing instance, and ensure that default routing instance has valid route to your syslog server.
Additionally, You can check with:
> monitor traffic interface <source_interface_of_your_log>
if any UDP packet destinated to your syslog server leaves interface.
Regards,