Thanks for posting your question in the Microsoft Q&A forum.
Unfortunately, Azure does not automatically invalidate or remove user-defined routes when the specified next hop becomes unreachable
UDRs are static routes that are manually configured and take precedence over system routes. They do not automatically update based on the reachability of the next hop.
Azure does not perform health checks or reachability tests on the next hop specified in a UDR.
Set up your own health monitoring system to check the availability of the NVA. If the NVA becomes unreachable, you can programmatically update or remove the UDR using Azure APIs or automation tools.
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