Welcome to the Microsoft Q&A Platform. Thank you for reaching out & I hope you are doing well.
This is an expected behavior.
- You cannot specify the nextHop type as ExpressRoute in vWAN Static Routing.
- See : Configuring static routes.
ExpressRoute, as a product, does not support such static routed traffic.
Instead, you should make use of BGP to advertise the address prefix and let the platform handle the routing with Any-to-any .
Hope this clarifies.
Cheers,
Kapil