Hi,
It seems that you are using Filter-based Forwarding , to choose between the two ISP conenctions. For this you might have a custom routing instance created with a route (staic probably) pointing to one of the ISPs and the same route in default instance (inet.0) pointing to the other ISP.
Depending on your firewall filter configuration ( match conditions), traffic coming on the interface (where you have applied this firewall filter) will be redirected to one of these two routing instances for route lookup.
When you add a route like this, this will be installed in inet.0 (default) instance.
set routing-options static route 204.248.140.0/24 next-hop 76.10.176.12
and when you do traceroute on the SRX, because this is a self traffic , it always take the route present in inet.0 .
If you do traceroute from your internal network, it will be entering SRX on your interface with firewall filter applied.
So depending on the filter match , it might take inet.0 route or custom routing-instance route. So please check your filter definition . That should give you an idea of why its taking a different path( firewall filter counters may help here) .