Something doesnt seem correct here
Are the 2 SRX's in an HA configuration?
If not, then i am not sure how you switch the firewalls over, but at the moment your configurations have
192.168.50.0/24 going to SRX A and 192.168.51.0/24 going to SRX B.
making SRX B active somehow, doesnt bring down tthe interfaces and therefore the routing will still go to SRXA.
The preference on a static route only means the route will become active if the primary route is removed from the table. This will only happen if an SLA tracker removes it or the interface goes down. Failing over does not bring this interface down, and therefore traffic wont use SRXB
If the SRX's are in HA configuration, you should be using reth groups and both SRX's will share the same IP address space.
