Since you have a cluster active passive, it means that only the 10.251.2.4 or 10.252.1.5 will pass traffic when each FGT is active/primary. I guess you might need to configure the vdom exceptions for the static routes so each FGT will have it's own static routes
The vdom exceptions allows you to set different values at some parts of your configuration that you specify in your cluster FGTs. Perhaps the AWS example in our documentation might be more clear than my explanation
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