The next hop address of default route of NE40 is 10.23.5.1 and mapping interface is vlanif12. When vlanif12 is down, default route is still valid, but the next hop interface is vlanif10.
Chekc route table of the system. There is a route with destination address as 10.0.0.0/8 and its next hop is interface vlanif10. Change destination address as 10.0.0.0/8 and ensure the route that does not contain 10.23.5.1. The default route is invalid.
The next hop of default route is 10.23.5.1. In route table, there is a route with destination address as 10.0.0.0/8 and the next hop interface as vlanif10. When vlanif12 interface is down, the default route can iterate 10.23.5.1 through destination address 10.0.0.0/8 because 10.0.0.0/8 contains 10.23.5.1. Default route is still valid, but the next hop is vlanif10.
Notice the influence of route iteration.