1. Interface is in security zone & hence the sessions is built in first place. So no doubt about zone/policies.
2. As per defination:
An interface’s primary address is used by default as the local address for broadcast and multicast packets sourced locally and sent out the interface.
An interface’s preferred address is the default local address used for packets sourced by the local router to destinations on the subnet.
3. You are trying to perform ping which is unicast & that too to a destination IP outside subnet.
4. We need to check routing to understand the behaviour.
5. Assist to grab output from the device : show route
6. Also One question to be answered: Are we only looking for self traffic generated by SRX towards internet or is this just for testing? As in, Is this question a minute question of a bigger question/problem that you are trying to fix/implement?
-Rahul