In case of routers (as well as for the general case) it's important to update the health of the ni/lpni for incoming messages. For an lpni specifically when we receive a message is when we know that the lpni is up.
A percentage router health is required in order to send a message to a gateway. That defaults to 100, meaning that a router interface has to be absolutely healthy in order to send to it. This matches the current behavior. So if a router interface goes down an its health goes down significantly, but then it comes back up again; either we receive a message from it or we discover it and get a reply, then in order to start using that router interface again we have to boost its health all the way up to maximum.
This behavior is special cased for routers.